Since I do book covers and formatting for other authors, in addition to my own, I get a lot of questions about whether or not to include images in ebooks, and whether those images need to be black and white.
The short answer is: Yes, put in as many pictures as you want (so long as you can compress them to the file size limit for your publisher of choice) and leave them in color.
Barns & Noble have the color Nook. Though Kindle may be in black and white, the kindle applications for PC, Mac, iPad, iPhone, and Android are in color. Not to mention that there are plenty of other brands of eReaders with color displays.
There is one thing to keep in mind, though, especially for technical diagrams. If readers use a gray scale device, such as kindle, the color image will be displayed in black and white. If you use images for tables, charts, etc you need to make sure that the colors have a good contrast so that readers can still distinguish the segments, or arrows, from one another. As for general illustrations, you’re probably fine.
Personally, I think it’s only a matter of time until all the ereaders are in color, and the black and white issue becomes a moot point. In the meantime, if you’re worried about how your images will appear in black and white, then try this “converter” – http://tjshome.com/imageconverter.php. Choose the black and white option (It’s the first one) then choose your image and hit the submit button. If you want you can right click on the results and save the converted image to your computer.