Rewrite TS minimap rendering:

* Rename LeftColor and RightColor to MinColor and MaxColor
  These are mapped from LowRadarColor and HighRadarColor in
  the original inis, and appear to be used to set the bounding
  values for selecting a random colour, NOT for left/right
  pixels (which caused noticeably wrong banding).

* Adjust brightness based on terrain height.
  MinHeightColorBrightness and MaxHeightColorBrightness
  were chosen by trial/error to match the original
  map preview rendering.

OpenRA.Mods.Common/UpdateRules/Rules/20200503/UpdateTilesetColors.cs

@@ -0,0 +1,50 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007-2020 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, either version 3 of
+ * the License, or (at your option) any later version. For more
+ * information, see COPYING.
+ */
+#endregion
+
+using System;
+using System.Collections.Generic;
+
+namespace OpenRA.Mods.Common.UpdateRules.Rules
+{
+	class UpdateTilesetColors : UpdateRule
+	{
+		public override string Name { get { return "Rename Tileset LeftColor and RightColor"; } }
+		public override string Description
+		{
+			get
+			{
+				return "The LeftColor and RightColor keys in tilesets have been renamed to MinColor and MaxColor to reflect their proper usage.";
+			}
+		}
+
+		public override IEnumerable<string> UpdateTilesetNode(ModData modData, MiniYamlNode tilesetNode)
+		{
+			if (tilesetNode.Key == "Templates")
+			{
+				foreach (var templateNode in tilesetNode.Value.Nodes)
+				{
+					foreach (var tilesNode in templateNode.ChildrenMatching("Tiles"))
+					{
+						foreach (var node in tilesNode.Value.Nodes)
+						{
+							foreach (var leftNode in node.ChildrenMatching("LeftColor"))
+								leftNode.RenameKey("MinColor");
+							foreach (var leftNode in node.ChildrenMatching("RightColor"))
+								leftNode.RenameKey("MaxColor");
+						}
+					}
+				}
+			}
+
+			yield break;
+		}
+	}
+}

OpenRA.Mods.Common/UpdateRules/UpdatePath.cs

@@ -65,6 +65,7 @@ namespace OpenRA.Mods.Common.UpdateRules
 				new SpawnActorPowerDefaultEffect(),
 				new RemoveConditionManager(),
 				new ConvertSupportPowerRangesToFootprint(),
+				new UpdateTilesetColors(),
 			})
 		};