Definitely true. The original sprites did their lighting effects by following the format of outline color, light shade, dark shade (i.e. the foot soldier is black outline, light purple, dark purple). The block colors i used were an attempt to get closer to the TMNT 2 and 3 sprites, but I was often limited by sprite size (since 2 and 3 are larger, they have the benefit of using an outline color for shadow as well). Oddly enough, the TMNT sprites dont really have the lighting effects, due to the need to have a dedicated bandana color, so i personally dont mind the block color enemies.
That said, that's just my opinion, so by all means, feel free to have at them! I wont have more time to really work on this other than small fixes going forward for awhile.