Move note glow to a separate class + make it match the design a bit more.

osu.Game.Rulesets.Mania/Objects/Drawables/Pieces/GlowPiece.cs

@@ -0,0 +1,62 @@
+using osu.Framework.Extensions.Color4Extensions;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Containers;
+using osu.Framework.Graphics.Shapes;
+using osu.Game.Graphics;
+using OpenTK.Graphics;
+
+namespace osu.Game.Rulesets.Mania.Objects.Drawables.Pieces
+{
+    public class GlowPiece : CompositeDrawable, IHasAccentColour
+    {
+        private const float glow_alpha = 0.7f;
+        private const float glow_radius = 5;
+
+        public GlowPiece()
+        {
+            RelativeSizeAxes = Axes.Both;
+            Masking = true;
+
+            InternalChild = new Box
+            {
+                RelativeSizeAxes = Axes.Both,
+                Alpha = 0,
+                AlwaysPresent = true
+            };
+        }
+
+        protected override void LoadComplete()
+        {
+            base.LoadComplete();
+            updateGlow();
+        }
+
+        private Color4 accentColour;
+        public Color4 AccentColour
+        {
+            get { return accentColour; }
+            set
+            {
+                if (accentColour == value)
+                    return;
+                accentColour = value;
+
+                updateGlow();
+            }
+        }
+
+        private void updateGlow()
+        {
+            if (!IsLoaded)
+                return;
+
+            EdgeEffect = new EdgeEffectParameters
+            {
+                Type = EdgeEffectType.Glow,
+                Colour = AccentColour.Opacity(glow_alpha),
+                Radius = glow_radius,
+                Hollow = true
+            };
+        }
+    }
+}