Quote Originally Posted by Niku-Sama View Post
sounds like a bad cap or a component getting too hot on that output in the shielded box. probably best to de solder the box and tap directly into the board. that way too you can feed a direct DC voltage too
Bad caps generally don't cause B&W pictures, loss of color data would but the fact he can tap the composite signal elsewhere suggests a cap isn't working correctly to begin with. My NES had bad caps that showed up as a lack of audio amplification. AT this point all of them likely have dried up in all consoles, replacing them is a solid start.

I'd scope the signal out to verify. The fact it starts in color is a good sign.