We have a blue-ray player. Yay us, right? It is hooked up to our TV via HDMI cable and this set up has been stable for years.
Just this week, it has suddenly wanted to show things only in black and white. WTF?
As we have NOT changed the cable set up that was working for a couple years the usual pat answer of “you hooked it up incorrectly” would not be applicable.
Any ideas what’s going on and/or how to fix it?
The first thing I would try is unhooking and reconnecting the HDMI cable at both ends. Try a different TV HDMI input if you have one. If that doesn’t help, if you have a second HDMI cable you can try, I would see if that cable works. Or connect a different device with the same cable. That could tell you if it’s the TV, the DVD or cable*.
A long shot maybe, but is there some TV or player setting that turns the color saturation up and down? Maybe some glitch set it to zero. It would have to be input-specific on the TV, or your regular TV would be black and white also.
If you are watching the original Wizard Of Oz, give it a few more minutes. 
As it’s an older blue ray player, it might be time for a new one. Fortunately the prices have fallen for them. Also if you hook up a new player and have the same problem, you’ll know the issue is probably the TV.
Another thought: are there other video outputs besides the HDMI? You could try to hook it up to the TV that way and see what happens.
Getting a new one is a great notion but I really can’t afford it. I’ve got much higher priorities like getting the muffler on my car fixed and new tires for the pickup and paying the rent.
Anyhow, it seems a particular “mode” got changed and that turned out to be the problem. Haven’t a clue how it got changed, probably hit the wrong button at some point. At least I didn’t get to the point of picking up the box, physically shaking it, and screaming “WTF IS WRONG WITH YOU?!?!?!” which isn’t particularly effective anyway.
As it happens, it first acted up that we were aware of during the play of our copy of The Day of the Doctor which also happens to start off in black and white.
Sell it as a “retro Blu-ray” to hipsters for $$$.
Buy a new one for <$$$.
Looks like it’s fixed, but ZenBeam’s advice would be what I’d’ve tried first.
I was going to suggest giving it a whack on the side after removing the disc.
The Day of the Doctor was very good as was The Time of the Doctor.
Seriously? Your Blu-ray player had a black & white only mode? I’ve never heard of that. But perhaps people want it for colorized movies.