They are in black ad white because that was what was frequently used for newspapers and stuff. Color film was often not as clear and the color tech they used was very hard to use with dark skin. So action shots of a march or speech ae just esier to do in black and white at the time.

Schools definitely like to pretend that its over and thats probably why they dont show any color pictures, as there were almost definitely color portraits of MLK and stuff, but the tech was actually a surprisingly limited factor as well.