Kieken & John: Short tube reflector, with focal length = 1400mm; not an f5.
Soldant, short tube + long focal length can only mean one thing: short focal length mirror with cheap barlow built into the scope (unless it cost thousands). These are meant to be the worst kind of cheap scopes. (Sorry)
Does this look familiar:
link to optrons 6"? If it does, then it's probably fishtank time.
My guess is that if initially the views were OK and now suddenly you are getting a lot of false colour, the built in barlow lens may gotten misaligned. Maybe you can rip it out and try using a real barlow, but it's a long shot.