I heard that you could turn off the colour and just use black and white, if so, how?

You would have to do it through your video card's drivers, I would think. I don't see an option in mine though. Why the heck would you want to, anyway?

As a joke?

Well, I never see computers monitors that have them, but I suppose if they did, you could do it at the monitor if they had color controls like the old TVs had on the front of them. Just 'dial back' the color until all it shows is shades of gray.

I wish you had mentioned this earlier. I finally got around to getting rid of a gazillion old computer system parts that I've been accumulating over the years. Two items were a monochrome monitor and an 8-bit ISA Hercules monochrome video adapter.

I read that it could be done in a comment thread on this Call of Duty mod that made most of the game look like old black and white movies.

That person was probably thinking of a demo that Valve did several months ago with Day of Defeat: Source. They were demonstrating several new graphics technologies that they were working on, one of which was an effect to make the game black & white and grainy looking, like an old movie. That was done through the game engine, though.

ati's smart shader will let you do so.

What about with a FX5200?