A client can convert a color image to black and white, but it's much more difficult to colorize a black and white image. We almost always recommend that you upload the color version of your content to maximize the possibility of sales, and we may send back black and white images to ask for the color version. It's your choice; you're welcome to resubmit the black and white if that is your artistic vision.
There are a few instances where black and white processing is absolutely incredible, and it would be hard for a client to reproduce. In that case, we recommend you upload the black and white version.
Content created using black and white film should, of course, be submitted in black and white.
Please don't upload both versions unless we've asked you to do so (generally based on a client request); pick the processing you feel is most suitable and submit that version.