Typical astronomical images have a very wide range of pixel values and generally, it is difficult to show the entire dynamical range in a color image. For example, by using ConvertType, it is possible to obtain a color image with three FITS images as each of the Red-Green-Blue (or RGB) color channels. However, depending on the pixel distribution, it could be very difficult to see the different regions together (faint and bright objects at the same time). In something like DS9, you end up changing the color map parameters to see the regions you are most interested in.
The reason is that images usually have a lot of faint pixels (near to the sky background or noise values), and few bright pixels (corresponding to the center of stars, galaxies, etc.) that can be millions of times brighter! As a consequence, by considering the images without any modification, it is extremely hard to visualize the entire range of values in a color image. This is because standard color formats like JPEG, TIFF or PDF are defined as 8-bit integer precision, while astronomical data are usually 32-bit floating point! To solve this issue, it is possible to perform some transformations of the images and then obtain the color image.
This is actually what the current script does: it makes some non-linear transformations and then uses Gnuastro’s ConvertType to generate the color image. There are several parameters and options in order to change the final output that are described in Invoking astscript-color-faint-gray. A full tutorial describing this script with actual data is available in Color images with full dynamic range. A general overview of this script is published in Infante-Sainz et al. 2024; please cite it if this script proves useful in your research.
GNU Astronomy Utilities 0.22 manual, February 2024.