If you are using an Svideo cable, try a new one or try a composite video cable first. Color information is sent separately in an S cable, yours could be bad.

Otherwise, unplug the DVD player from power and wait a couple minutes before plugging it back in.

Last non-technical resort: Go shopping for a replacement, unless it's an expensive unit and worth repairing. Problems in the chroma circuit could cost $75 or more to repair.