Add OsuColour method mapping colours from basic theme to mod types

osu.Game/Graphics/OsuColour.cs

@@ -5,6 +5,7 @@ using System;
 using osu.Framework.Extensions.Color4Extensions;
 using osu.Game.Beatmaps;
 using osu.Game.Overlays;
+using osu.Game.Rulesets.Mods;
 using osu.Game.Rulesets.Scoring;
 using osu.Game.Scoring;
 using osu.Game.Utils;
@@ -157,6 +158,36 @@ namespace osu.Game.Graphics
             }
         }
 
+        /// <summary>
+        /// Retrieves the main accent colour for a <see cref="ModType"/>.
+        /// </summary>
+        public Color4 ForModType(ModType modType)
+        {
+            switch (modType)
+            {
+                case ModType.Automation:
+                    return Blue1;
+
+                case ModType.DifficultyIncrease:
+                    return Red1;
+
+                case ModType.DifficultyReduction:
+                    return Lime1;
+
+                case ModType.Conversion:
+                    return Purple1;
+
+                case ModType.Fun:
+                    return Pink1;
+
+                case ModType.System:
+                    return Gray7;
+
+                default:
+                    throw new ArgumentOutOfRangeException(nameof(modType), modType, "Unknown mod type");
+            }
+        }
+
         /// <summary>
         /// Returns a foreground text colour that is supposed to contrast well with
         /// the supplied <paramref name="backgroundColour"/>.