Implement new difficulty rating colour spectrum sampling

osu.Game/Graphics/OsuColour.cs

@@ -69,6 +69,26 @@ namespace osu.Game.Graphics
             }
         }
 
+        public Color4 ForStarDifficulty(double starDifficulty)
+        {
+            var spectrumPoint = (float)(starDifficulty / 8.00);
+
+            if (spectrumPoint > 1f)
+                return Color4.Black;
+
+            return ColourUtils.SampleFromLinearGradient(new[]
+            {
+                (0.2159f, Color4Extensions.FromHex("4fc0ff")),
+                (0.2693f, Color4Extensions.FromHex("4fffd5")),
+                (0.3217f, Color4Extensions.FromHex("7cff4f")),
+                (0.4111f, Color4Extensions.FromHex("f6f05c")),
+                (0.5767f, Color4Extensions.FromHex("ff8068")),
+                (0.7307f, Color4Extensions.FromHex("ff3c71")),
+                (0.8667f, Color4Extensions.FromHex("6563de")),
+                (0.9996f, Color4Extensions.FromHex("18158e")),
+            }, spectrumPoint);
+        }
+
         /// <summary>
         /// Retrieves the colour for a <see cref="ScoreRank"/>.
         /// </summary>

osu.Game/Utils/ColourUtils.cs

@@ -0,0 +1,37 @@
+// 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 System.Collections.Generic;
+using osu.Framework.Utils;
+using osuTK.Graphics;
+
+namespace osu.Game.Utils
+{
+    public static class ColourUtils
+    {
+        /// <summary>
+        /// Samples from a given linear gradient at a certain specified point.
+        /// </summary>
+        /// <param name="gradient">The gradient, defining the colour stops and their positions (in [0-1] range) in the gradient.</param>
+        /// <param name="point">The point to sample the colour at.</param>
+        /// <returns>A <see cref="Color4"/> sampled from the linear gradient.</returns>
+        public static Color4 SampleFromLinearGradient(IReadOnlyList<(float position, Color4 colour)> gradient, float point)
+        {
+            if (point < gradient[0].position)
+                return gradient[0].colour;
+
+            for (int i = 0; i < gradient.Count - 1; i++)
+            {
+                var startStop = gradient[i];
+                var endStop = gradient[i + 1];
+
+                if (point >= endStop.position)
+                    continue;
+
+                return Interpolation.ValueAt(point, startStop.colour, endStop.colour, startStop.position, endStop.position);
+            }
+
+            return gradient[^1].colour;
+        }
+    }
+}