Examine the serial cable. If any of the pins are bent, this can cause the printer to think that a special loopback test plug is installed. The loopback plug places the printer into an extended diagnostic mode which produces the three color page you have described. This also happens on the Color LaserWriter 12/660 PS printer.
Power down the printer, then remove the serial cable. Then power the printer back on. You should not see the three color diagnostic page. If removing the cable solves this issue, you need to replace the cable, or carefully bend the pins back into position. However, if after removing the cable you are still getting the three color diagnostic page, you should get your Color LaserWriter 12/600 PS printer serviced.