Codechange: Store Colours in Colours type. (#11625)

This reduces casts, some magic numbers, and introduces a bit of type-safety.

src/saveload/company_sl.cpp

@@ -516,7 +516,7 @@ struct PLYRChunkHandler : ChunkHandler {
 		while ((index = SlIterateArray()) != -1) {
 			Company *c = new (index) Company();
 			SlObject(c, slt);
-			_company_colours[index] = (Colours)c->colour;
+			_company_colours[index] = c->colour;
 		}
 	}