Color establishes a renewed familiarity with the past.
There is an element of detachment that we have from historic black and white images. It's as if they are only shadows from a time too long ago for any of us to remember. With our modern eyes, we are somehow disconnected from the real and vibrant world those photos are actually portraying. By adding color to these images of history, the viewer is brought a little closer to the reality in which they were taken. Color helps to give a little bit of a glimpse into the world as it was from long ago, an opportunity to see perhaps something like what the photographer himself saw through his lens.
Color can force us to instantly see an old photograph with a new perspective, and make it seem as if the past it portrays wasn't that long ago after all.