Got up early to shut the scope down from imaging the previous night.
Had another go at Saturn with the usual setup (DMK for luminance, SPC900NC for colour). Again I combined shots of the planet in CCDStack and I'm happy with the result.
Undoubtedly Mike, I'm sure you have the same problem when using the RGB filters, the blue channel is always rubbish. I can't remember seeing violet before though, might have a really good look at the subs, see if something is misaligned on the colour ones.