I'm not a genius, especially with the RPG Maker system, but if you have problems due to color or whatever go into photoshop or paintshop and make sure it's an 8-bit 256-color .png file. To my knowledge RM2k3 doesn't accept anything else but .XYZ files, which cann't be created, as far as I know, in photoshop.
So just double-check the colors and try again. If there's still a problem maybe someone else can help you more. o.o