diff --git a/src/lua-engine.cpp b/src/lua-engine.cpp
index b337513b3..44f88346e 100644
--- a/src/lua-engine.cpp
+++ b/src/lua-engine.cpp
@@ -3745,8 +3745,7 @@ enum
  * ourselves.
  */
 static uint8 gui_colour_rgb(uint8 r, uint8 g, uint8 b) {
-	static uint8 index_lookup[1 << (3+3+3)];
-	int k;
+	static uint8 index_lookup[1 << (5+5+5)];
 
 	if (!gui_saw_current_palette)
 	{
@@ -3754,22 +3753,23 @@ static uint8 gui_colour_rgb(uint8 r, uint8 g, uint8 b) {
 		gui_saw_current_palette = TRUE;
 	}
 
-	k = ((r & 0xE0) << 1) | ((g & 0xE0) >> 2) | ((b & 0xE0) >> 5);
+    // Cache based on upper 5 bits of r, g, and b
+	int k = ((r & 0xF8) << 7) | ((g & 0xF8) << 2) | ((b & 0xF8) >> 3);
+    if (index_lookup[k] != GUI_COLOUR_CLEAR) return index_lookup[k];
 	uint16 test, best = GUI_COLOUR_CLEAR;
-	uint32 best_score = 0xffffffffu, test_score;
-	if (index_lookup[k] != GUI_COLOUR_CLEAR) return index_lookup[k];
+	uint32 test_score, best_score = 0xffffffffu;
 	for (test = 0; test < 0xff; test++)
 	{
 		uint8 tr, tg, tb;
 		if (test == GUI_COLOUR_CLEAR) continue;
 		FCEUD_GetPalette(test, &tr, &tg, &tb);
-		test_score = abs(r - tr) *  66 +
-		             abs(g - tg) * 129 +
-		             abs(b - tb) *  25;
+        // Basic distance squared
+		test_score = (r - tr) * (r - tr) +
+                     (g - tg) * (g - tg) +
+                     (b - tb) * (b - tb);
 		if (test_score < best_score) best_score = test_score, best = test;
 	}
-	index_lookup[k] = best;
-	return best;
+	return index_lookup[k] = best;
 }
 
 void FCEU_LuaUpdatePalette()