This page compares different methods for extracting luminance data from an LRGB or RGB image. The Original LRGB was created with PixInsight. A 64-bit PixInsight luminance file was combined with a CCDStack 32-bit color FITS image. The LRGB image was saved as a 16-bit TIF, with no color profile, for loading into PhotoShop.

The reason for creating the color in CCDStack is another story.

The statistical data are for the background, as determined by PhotoShop.  Note that the original image has a much lower signal-to-noise ratio. This is probably due to variable background color.

From the numbers, it looks like the best way to extract the luminous data is to convert the color image to grayscale with PhotoShop.