In many cases, a DVD plays in black and white because the cables linking the player to the TV are improperly connected or defective. However, there are a number of other issues that cause a DVD to play in black and white instead of color.
If the DVD is connected using A/V cables, check to see if the white, red and yellow plugs are properly connected to the same colored jacks on both the television and DVD player. If the DVD player is connected with component cables, make sure that the red, blue and green cables are securely connected to the same colored jacks on both the DVD player and TV. Similarly, if the two are connected using an S-video cable, make sure that both ends of the cable are properly connected.
If the cables are properly connected and the problem still persists, try to play a different DVD to see if the problem is related to the disc. If the second DVD also plays in black and white, the next step is to replace the cables, as they may be defective. If the DVD player is connected via an A/V receiver, it may also be necessary to replace the cable connecting the receiver to the TV.
If this still doesn't solve the problem, try connecting the cables to a different jack to determine whether one of the jacks is defective. If none of these things work, it may be necessary to have the DVD player serviced.