It isn't clear if the picture just loses color or the screen goes blank. If you are just losing color (picture goes to B/W) there is a problem in the color circuit and from your description it could easily be a bad solder connection on the board which decodes the color portion of the picture. If you are capable with a soldering iron and know TV HV safety, unplug the set, and remove the circuit boards and inspect (with a high powered magnifier)and resolder all suspect looking connections on the boards. You could also resolder all the connections which is more work but assures that solder joints are not the problem. If your problem is not solved at this point you are probably going to need a technician to look at it as it could be just about anything and probably would be beyond your expertise. Good luck and be safe.
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