Someone said that pencil in artwork is the same as piano in music. It's a base for everything. And for unknown reason I like it best of all. So as the black&white pictures and retro photos. When the only color is gray, nothing pushes your imagination, and you have more freedom imaging any color you like.
Most of my handdrawn pictures were made with pencil only. They're black and white, but it doesn't mean that I don't like color.
When I want to add some color to the picture, I prefer to use computer for it. I scan pencil drawn sketch and begin experimenting...
What I noticed after uploading both black&white pencil drawings and their colored variants to DT is that colored pictures sell better, and sell fast! Sometimes even after a few hours being online when sketches have to wait for a buyer much longer.

So I come to conclusion: it's a good idea to make colored variant for every pencil picture. Several different color variants and maybe some vector ones would do only better.