> You can put them in to black-and-white mode and their viewfinders show black and white. That's a great way for an amateur photog to learn to see light unconfused by color.

FWIW, I think this is a bad idea in general. For digital photography, black and white conversion should be done in post, where you can both keep the color information in case you need it later, and where you can play around with setting different luminosity settings for different colors of light.

Playing around with that will give you a much better idea of what sorts of black and white images you can turn a color image into.

Here are a couple examples of the different effects you can get from the same color source:

http://imgur.com/a/xDrkj

Both images are from the same source; one was edited to simulate a red filter on the lens; one was used to simulate a blue filter. As you can see, the effect is profoundly different; using an in-camera "black and white" mode gives up the ability to learn those differences.