It might be that the wire color limit is for the same reason that the current engine is restricted to 4 liquids. There's only 2 bits that can be used for liquids and very likely only 2 bits for wire colors in the engine and you can only get 4 possibilities with 2 bits (00, 01, 10, 11). That sounds like the main reason. They could add a 3rd Bit to get 8 possible liquids or Wire Colors, but it would take a long time for them since it sounds like they would have to modify the engine itself. The Liquid limit was officially confirmed by Yoraiz0r here.