Greg,
You can colorize all the photos you want, as long as you took them or have permission from the photographer to alter them. You are correct, it was a popular techinque before color photography was developed. But it was done by those who had vested commercial interest in their own images or with the permission of family members, and done before the current copyright laws. Public domain or not, mutilating old photos for fun or profit by any sort of alteration changes the origional artistic or historical intent of the image maker. There is simply no practial need to destroy the work of others in this way. I, for one, have no trouble visualizing what a yellow coach looks like. A sample color / good description / reference is all I need to make my models come into "historical" life. Gary