When an image exits the stacking process, it is "linear." In this state, the pixel values are directly proportional to the number of photons captured. This is physically accurate but visually abysmal to the human eye—appearing jet-black save for a few faint star cores. More critically, in this linear stage, the background sky glow (light pollution or airglow) dominates the dim signal of nebulae and galaxies. If a photographer applies a simple stretch without linking the channels, the channel with the highest background noise (often the blue or red channel, depending on the sky conditions) will balloon out of control, resulting in a green or magenta cast across the entire image.
PixInsight applies the same mathematical stretch to all three color channels (Red, Green, and Blue) based on the image's overall statistics. pixinsight lerar link
Once the LRGB combination is complete, the image usually looks good, but it can be improved. When an image exits the stacking process, it is "linear