The 'cluster' itself Danny, is pretty good, with nice granular looking stars.
You don't say what software that you are aligning/stacking your images with, and if you are using 'auto' or 'manual' star alignment?
I have seen this effect myself, when I have captured the RGB subs during different imaging sessions, and its usually caused where the rotational position of the camera has shifted slightly from session to session.
I always align manually on two stars, and try to use the same two stars in each sub.
Theoretically, the alignment algorithm should alway use the centroid of the star, but it doesn't need to be very much adrift to misalign the colour planes.
As I said, I've only ever seen this where rotational alignment has been involved.
Meade 8" LX200ACF, WO FLT 110, SXVR-H16, SXVF-H9, SX Lodestar, DMK21,Meade LPI, NEQ6 Pro Synscan mount, and Observatory.
Web Site: davesastronomy/