It looks typical of a plating montipora, nothing to worry about. Usually the edges, the new growth will be different color. Fragging it, is your choice. The coral is growing, plating looking for a new edge to attach too. Also your monti does not look like a rainbow monti, more of a plating monti. Rainbow monti is an encrusting monti. They are usually bluish base body with green polyps. The edges or new growth will have orange, green, and yellow polyps. A small frag will have a nice rainbow of assorted polyps because its so small and almost all of it is new growth.
Youe coral base is for sure purple all the way with green polyps...plating coral I believe.. Its a nice piece, but I don't know the name of it.