Move out into a separate method

osu.Game.Rulesets.Catch/Skinning/CatchLegacySkinTransformer.cs

@@ -66,7 +66,7 @@ namespace osu.Game.Rulesets.Catch.Skinning
                     if (result == null)
                         return null;
 
-                    result.Value = result.Value.ToLegacyColour();
+                    result.Value = LegacyColourCompatibility.DisallowZeroAlpha(result.Value);
                     return (IBindable<TValue>)result;
             }
 

osu.Game.Rulesets.Catch/Skinning/LegacyFruitPiece.cs

@@ -75,7 +75,7 @@ namespace osu.Game.Rulesets.Catch.Skinning
         {
             base.LoadComplete();
 
-            accentColour.BindValueChanged(colour => colouredSprite.Colour = colour.NewValue.ToLegacyColour(), true);
+            accentColour.BindValueChanged(colour => colouredSprite.Colour = LegacyColourCompatibility.DisallowZeroAlpha(colour.NewValue), true);
         }
     }
 }

osu.Game.Rulesets.Mania/Skinning/LegacyColumnBackground.cs

@@ -58,14 +58,20 @@ namespace osu.Game.Rulesets.Mania.Skinning
 
             InternalChildren = new Drawable[]
             {
-                new Box { RelativeSizeAxes = Axes.Both }.WithLegacyColour(backgroundColour),
+                LegacyColourCompatibility.ApplyWithDoubledAlpha(new Box
+                {
+                    RelativeSizeAxes = Axes.Both
+                }, backgroundColour),
                 new Container
                 {
                     RelativeSizeAxes = Axes.Y,
                     Width = leftLineWidth,
                     Scale = new Vector2(0.740f, 1),
                     Alpha = hasLeftLine ? 1 : 0,
-                    Child = new Box { RelativeSizeAxes = Axes.Both }.WithLegacyColour(lineColour)
+                    Child = LegacyColourCompatibility.ApplyWithDoubledAlpha(new Box
+                    {
+                        RelativeSizeAxes = Axes.Both
+                    }, lineColour)
                 },
                 new Container
                 {
@@ -75,7 +81,10 @@ namespace osu.Game.Rulesets.Mania.Skinning
                     Width = rightLineWidth,
                     Scale = new Vector2(0.740f, 1),
                     Alpha = hasRightLine ? 1 : 0,
-                    Child = new Box { RelativeSizeAxes = Axes.Both }.WithLegacyColour(lineColour)
+                    Child = LegacyColourCompatibility.ApplyWithDoubledAlpha(new Box
+                    {
+                        RelativeSizeAxes = Axes.Both
+                    }, lineColour)
                 },
                 lightContainer = new Container
                 {
@@ -86,7 +95,7 @@ namespace osu.Game.Rulesets.Mania.Skinning
                     {
                         Anchor = Anchor.BottomCentre,
                         Origin = Anchor.BottomCentre,
-                        Colour = lightColour.ToLegacyColour(),
+                        Colour = LegacyColourCompatibility.DisallowZeroAlpha(lightColour),
                         Texture = skin.GetTexture(lightImage),
                         RelativeSizeAxes = Axes.X,
                         Width = 1,

osu.Game.Rulesets.Mania/Skinning/LegacyHitTarget.cs

@@ -56,7 +56,7 @@ namespace osu.Game.Rulesets.Mania.Skinning
                         Anchor = Anchor.CentreLeft,
                         RelativeSizeAxes = Axes.X,
                         Height = 1,
-                        Colour = lineColour.ToLegacyColour(),
+                        Colour = LegacyColourCompatibility.DisallowZeroAlpha(lineColour),
                         Alpha = showJudgementLine ? 0.9f : 0
                     }
                 }

osu.Game.Rulesets.Osu/Skinning/LegacyMainCirclePiece.cs

@@ -106,7 +106,7 @@ namespace osu.Game.Rulesets.Osu.Skinning
             base.LoadComplete();
 
             state.BindValueChanged(updateState, true);
-            accentColour.BindValueChanged(colour => hitCircleSprite.Colour = colour.NewValue.ToLegacyColour(), true);
+            accentColour.BindValueChanged(colour => hitCircleSprite.Colour = LegacyColourCompatibility.DisallowZeroAlpha(colour.NewValue), true);
             indexInCurrentCombo.BindValueChanged(index => hitCircleText.Text = (index.NewValue + 1).ToString(), true);
         }
 

osu.Game.Rulesets.Osu/Skinning/LegacySliderBall.cs

@@ -39,11 +39,11 @@ namespace osu.Game.Rulesets.Osu.Skinning
                     Texture = skin.GetTexture("sliderb-nd"),
                     Colour = new Color4(5, 5, 5, 255),
                 },
-                animationContent.WithLegacyColour(ballColour).With(d =>
+                LegacyColourCompatibility.ApplyWithDoubledAlpha(animationContent.With(d =>
                 {
                     d.Anchor = Anchor.Centre;
                     d.Origin = Anchor.Centre;
-                }),
+                }), ballColour),
                 layerSpec = new Sprite
                 {
                     Anchor = Anchor.Centre,

osu.Game.Rulesets.Taiko/Skinning/LegacyCirclePiece.cs

@@ -90,7 +90,7 @@ namespace osu.Game.Rulesets.Taiko.Skinning
 
         private void updateAccentColour()
         {
-            backgroundLayer.Colour = accentColour.ToLegacyColour();
+            backgroundLayer.Colour = LegacyColourCompatibility.DisallowZeroAlpha(accentColour);
         }
     }
 }

osu.Game.Rulesets.Taiko/Skinning/LegacyDrumRoll.cs

@@ -76,9 +76,9 @@ namespace osu.Game.Rulesets.Taiko.Skinning
 
         private void updateAccentColour()
         {
-            headCircle.AccentColour = accentColour.ToLegacyColour();
+            headCircle.AccentColour = LegacyColourCompatibility.DisallowZeroAlpha(accentColour);
-            body.Colour = accentColour.ToLegacyColour();
+            body.Colour = LegacyColourCompatibility.DisallowZeroAlpha(accentColour);
-            end.Colour = accentColour.ToLegacyColour();
+            end.Colour = LegacyColourCompatibility.DisallowZeroAlpha(accentColour);
         }
     }
 }

osu.Game.Rulesets.Taiko/Skinning/LegacyHit.cs

@@ -19,9 +19,10 @@ namespace osu.Game.Rulesets.Taiko.Skinning
         [BackgroundDependencyLoader]
         private void load()
         {
-            AccentColour = (component == TaikoSkinComponents.CentreHit
+            AccentColour = LegacyColourCompatibility.DisallowZeroAlpha(
-                ? new Color4(235, 69, 44, 255)
+                component == TaikoSkinComponents.CentreHit
-                : new Color4(67, 142, 172, 255)).ToLegacyColour();
+                    ? new Color4(235, 69, 44, 255)
+                    : new Color4(67, 142, 172, 255));
         }
     }
 }

osu.Game/Skinning/LegacyColourCompatibility.cs

@@ -0,0 +1,46 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using osu.Framework.Graphics;
+using osuTK.Graphics;
+
+namespace osu.Game.Skinning
+{
+    /// <summary>
+    /// Compatibility methods to convert osu!stable colours to osu!lazer-compatible ones. Should be used for legacy skins only.
+    /// </summary>
+    public static class LegacyColourCompatibility
+    {
+        /// <summary>
+        /// Forces an alpha of 1 if a given <see cref="Color4"/> is fully transparent.
+        /// </summary>
+        /// <remarks>
+        /// This is equivalent to setting colour post-constructor in osu!stable.
+        /// </remarks>
+        /// <param name="colour">The <see cref="Color4"/> to disallow zero alpha on.</param>
+        /// <returns>The resultant <see cref="Color4"/>.</returns>
+        public static Color4 DisallowZeroAlpha(Color4 colour)
+        {
+            if (colour.A == 0)
+                colour.A = 1;
+            return colour;
+        }
+
+        /// <summary>
+        /// Applies a <see cref="Color4"/> to a <see cref="Drawable"/>, doubling the alpha value into the <see cref="Drawable.Alpha"/> property.
+        /// </summary>
+        /// <remarks>
+        /// This is equivalent to setting colour in the constructor in osu!stable.
+        /// </remarks>
+        /// <param name="drawable">The <see cref="Drawable"/> to apply the colour to.</param>
+        /// <param name="colour">The <see cref="Color4"/> to apply.</param>
+        /// <returns>The given <paramref name="drawable"/>.</returns>
+        public static T ApplyWithDoubledAlpha<T>(T drawable, Color4 colour)
+            where T : Drawable
+        {
+            drawable.Alpha = colour.A;
+            drawable.Colour = DisallowZeroAlpha(colour);
+            return drawable;
+        }
+    }
+}

osu.Game/Skinning/LegacySkinExtensions.cs

@@ -9,7 +9,6 @@ using osu.Framework.Graphics.Animations;
 using osu.Framework.Graphics.OpenGL.Textures;
 using osu.Framework.Graphics.Sprites;
 using osu.Framework.Graphics.Textures;
-using osuTK.Graphics;
 using static osu.Game.Skinning.LegacySkinConfiguration;
 
 namespace osu.Game.Skinning
@@ -63,36 +62,6 @@ namespace osu.Game.Skinning
             }
         }
 
-        /// <summary>
-        /// The resultant colour after setting a post-constructor colour in osu!stable.
-        /// </summary>
-        /// <param name="colour">The <see cref="Color4"/> to convert.</param>
-        /// <returns>The converted <see cref="Color4"/>.</returns>
-        public static Color4 ToLegacyColour(this Color4 colour)
-        {
-            if (colour.A == 0)
-                colour.A = 1;
-            return colour;
-        }
-
-        /// <summary>
-        /// Equivalent of setting a colour in the constructor in osu!stable.
-        /// Doubles the alpha channel into <see cref="Drawable.Alpha"/> and uses <see cref="ToLegacyColour"/> to set <see cref="Drawable.Colour"/>.
-        /// </summary>
-        /// <remarks>
-        /// Beware: Any existing value in <see cref="Drawable.Alpha"/> is overwritten.
-        /// </remarks>
-        /// <param name="drawable">The <see cref="Drawable"/> to set the colour of.</param>
-        /// <param name="colour">The <see cref="Color4"/> to set.</param>
-        /// <returns>The given <see cref="Drawable"/>.</returns>
-        public static T WithLegacyColour<T>(this T drawable, Color4 colour)
-            where T : Drawable
-        {
-            drawable.Alpha = colour.A;
-            drawable.Colour = ToLegacyColour(colour);
-            return drawable;
-        }
-
         public class SkinnableTextureAnimation : TextureAnimation
         {
             [Resolved(canBeNull: true)]