Why is the night sky photo showing uneven color and strange pattern in background?

Question

Here's straight-out-of-camera JPEG I've shot with Fujifilm XT-2 with following details:

f/2.8
15 seconds
ISO 3200
XF 16mm f/2.8 R WR

Upon closer inspection I noticed:

Left area is noisy possibly due to bumped ISO with artificial light

Some area is blotchy with uneven color patches all over the places

Magnified @ 100% on left area (saturation +100% to show orangish and greenish hue)

Magnified @ 100% on right area (boosted luminosity to show uneven pattern in background)

May I know what caused these artifacts and how do I avoid it while shooting night sky? Thank you in advance!
P/S: I understand shooting RAW will definitely have advantage in this case but currently I don't have a software to process RAF yet. Original file uploaded to filebin.

Kai Mattern · Accepted Answer

Colors of the stars
Very small light sources are a problem with the rendering of color from the sensor.  Basically the star itself is not much bigger than one pixel, but you will light to some red, green and blue pixels. This can lead to the color of the stars going off, depending how many of the rgb pixels of the sensor get some light from it.
On larger light sources, the color would even out, but here a shift can occur.
Then, with small light sources, chromatic aberations might become more distinct around the lights, affecting the color as well.
A last effect is high iso noise, why can also contribute.
Color Patch
This sees to be light pollution from dust in the atmosphere which lights up with the lights of cities nearby.

Why is the night sky photo showing uneven color and strange pattern in background?

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