The Sega Genesis stores color values in a 16-bit data type with 3 significant bits for each color channel. 0x00 should be the darkest, and 0x07 should be the brightest. What those values translate to depends on your output device.
Genecyst uses a less accurate method of color conversion, its screenshots are darker
Gens uses a more accurate method for the 16-bit screen output, however its screenshot function bit-shifts those values into 24-bit, so a Print Screen screenshot and screenshot from the emulator will produce different colors on most machines.
If you're using the Kmod extension of Gens, its tile and sprite tools also use a less accurate method of color conversion.
The color difference isn't usually noticeable to the eye, until you start mixing pieces from different ripping methods.
Genecyst uses a less accurate method of color conversion, its screenshots are darker
Gens uses a more accurate method for the 16-bit screen output, however its screenshot function bit-shifts those values into 24-bit, so a Print Screen screenshot and screenshot from the emulator will produce different colors on most machines.
If you're using the Kmod extension of Gens, its tile and sprite tools also use a less accurate method of color conversion.
The color difference isn't usually noticeable to the eye, until you start mixing pieces from different ripping methods.