Problem with colour gradients(?) on Mosaic

[tomato]

Last October I managed a 6 panel mosaic of M31 in a single night, 30 mins L, and just 10 mins each on RGB on each panel. I had a lot of trouble neutralising the background in the Lum channel, the top RH panel was noticeably different to the rest, I suspected some high cloud on these subs at the time. 

Anyway, I have took more time with the settings in APP and have made some progress on this problem, but now I have combined the RGB channels, another one has arisen. For some reason the LH two panels have a blue cast, the centre two a magenta cast and the RH two are decidedly green. I am putting this down to the very limited colour data available but why it would manifest itself like this I don't know. Is there any way to fix it? I now have PI and Affinity Photo available, but I'm not sure how to approach it.

Another thought, I havn't cropped the image as I didn't want to lose the companion galaxy, but I'm wondering if this is the root cause? Can you crop an image using a non rectangular template?

[MarkAR]

Wonderful image.

Not sure what you can do about the cropping.

For the gradients/colour cast you can try DBE in PixInsight, maybe Background neutralisation and a few other tweaks.

[tomato]

Thanks, I will try the tools in PI,  does it make sense to apply DBE to the LRGB channels separately, then combine?

[MarkAR]

Not sure, I've done it a few times but it depends on each image. 

[tomato]

I had a go with Startools 1.6, it took quite a while to crunch an image of this size, but I think it is an improvement?

 

[alexbb]

If you have data covering the whole (or even wider) area, check this out: https://pixinsight.com/tutorials/multiscale-gradient-correction/

[Laurin Dave]

Given the very limited data you have in this I think you’ve done a great job here.  I’ve tried some big mosaics and have had the same issues..  lots of data helps as does ruthless culling of poor data ..  I ended up processing each panel separately matching background levels before combining in APP..  Still wasn’t entirely happy so will capture more data next time the targets come round ..  as for cropping if you have Photoshop or Afinity does layers then you can create an image with a rectangular crop out in the lower left by cropping your base image,  duplicating it a couple of times cropping the duplicates to suit then pasting them back over the cropped base image and using the move tool to align them .. reduce opacity in the overlying layers for alignment then increase back to 100% flatten and you have it..  

Dave

[tomato]

Thanks for the help, it was never going to be easy with so little data, I really need 6 hours on each panel. 
Still, the dual rig is available now so hopefully I can improve on this when Autumn rolls around.