Implement IAccented interface + transform.

osu.Game/Graphics/Transforms/TransformAccent.cs

@@ -0,0 +1,37 @@
+// Copyright (c) 2007-2017 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using OpenTK.Graphics;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Transforms;
+using osu.Framework.MathUtils;
+
+namespace osu.Game.Graphics.Transforms
+{
+    public class TransformAccent : Transform<Color4>
+    {
+        /// <summary>
+        /// Current value of the transformed colour in linear colour space.
+        /// </summary>
+        public override Color4 CurrentValue
+        {
+            get
+            {
+                double time = Time?.Current ?? 0;
+                if (time < StartTime) return StartValue;
+                if (time >= EndTime) return EndValue;
+
+                return Interpolation.ValueAt(time, StartValue, EndValue, StartTime, EndTime, Easing);
+            }
+        }
+
+        public override void Apply(Drawable d)
+        {
+            base.Apply(d);
+
+            var accented = d as IAccented;
+            if (accented != null)
+                accented.AccentColour = CurrentValue;
+        }
+    }
+}