I think you'll have to paint -- color goes right through with bricks...

Before you paint the brick, remember that anything painted inevitably needs to be repainted and layers of built up paint get ugly.

Instead, find some old clothes you don't mind ruining and protect the floor with a painter's thick plastic drop cloth. If you have some old towels or rugs to put atop that to catch water, do.

...First, scrub the brick with a scrub brush. You might first try the spray on foam bathroom tile cleaners but you'll have less lather to sponge off/up if you use one of those powdered cleaners from a cardboard can about the size of a can of soda (bon ami or ajax or comet) and they have grit in them to help with the cleaning.

...Second, put some of that liquid bleach that's thick -- designed to not splash much -- in a shallow dispan. Continuing to protect your floor and remembering to protect your eyes, use a broom with dust bristles on the ends of the straw to apply the bleach to the entire brick wall and let it dry in; repeat at least once.

Now you can actually see what kind of brick wall you have to work with. You may decide to keep it as is.