SDSS has an interesting algorithm to merge images from three filters into an RGB-colored image (for example JPEG or PDF).

The algorithm is defined in Lupton et al. 2004, with the goal of showing the brightest parts of an image in logarithmic scale and the fainter parts in linear scale (to show both).

An implementation of this is in Astropy, as the  make_lupton_rgb function.

Fortunately implementing the basic function is easy in ConvertType to avoid the huge dependency tree of Astropy and do it fast! It can be implemented in an option like the example below. Where AAA is the Q parameter and BBB is the "stretch" parameter.

astconvertt red.fits green.fits blue.fits --luptonrgb=AAA,BBB

Raul Infante-Sainz kindly provided the links above and this issue was recommended by Ignacio Trujillo.