Why are manga black and white? This question triggered my curiosity for this question. Why most of manga are black and white? Is this because of more cost if they add colors to it?
There can be different reasons for that, let's try to imagine some of them. 


Lower cost. This is obvious (just compare the prices for black ink cartridges and color cartridges for your printer to see the difference). Also, consider that the lower cost of production causes lower prices for the end product - so the readers will be more eager to buy manga.
Faster production. Unlike comics in the US, for example, which usually comes out on a monthly basis, a lot of manga comes out weekly. Coloring would take additional time and make it harder to release new chapters in time.
Manga artists usually don't have a lot of staff to help them, and sometimes even work alone. Thus, they don't have enough time (and, not rarely, enough skill) to color their works well, because:


Coloring is completely different from line art. Not all people that can draw good line art can color well, and vice versa. If you talk about comics, there are usually at least two artists making them: one artist that draws line art, and another artist that is responsible for coloring (this artist is called a colorist). Believe me, coloring is hard. And of course, that person also needs to be paid :P

It also somehow raises the interest of manga fans in watching anime (which is in color), but I doubt this is really a valuable reason, so think of it as of an interesting observation.