Messier 15

Acquisition:
Bortle 6-7 (some 2-3) skies
Nikon D7000 camera
Orion Apex 102mm telescope, 1300mm focal length
Orion AstroView EQ3 with single-axis drive
30s x 34 frames (20 frames from Bortle 2-3)
f/12.7
ISO 1600

Processing:
Siril:

Photometric color calibration
Background extraction
Asinh histogram stretch
GIMP:

Decompose RGB to layers
Iain' Noise Reduction (G'MIC-Qt plugin) on red and blue layers
Compose layers to new image
Color value curve adjustment
Slight saturation increase
RawTherapee:

Exposure compensation increase (for general brightness)
Highlight compression (to keep center from blowing out)