In my experience, no, a prism diagonal will not reduce the color fringing in a fast achromat. I have one achromat, 80/F7 where it seemed to do the reverse, it put a blue color hue across the lunar disc as if I had put a light blue filter on the eyepiece. But I think Clive has hit on something here. If you have a long achromat like the Stellarvue 80/9D or Burgess Planet Hunter, a prism diagonal may be a "forgotten" accessory for these scopes.

David E