If you were talking about bmp2scm, I'm pretty sure its:
Find the tile you want to use in StarForge. At the top, it has 2 numbers [xxx - xx]. Multiply the first number by 16 and add the second number, and replace it with the corresponding color.
It's much more complicated than SCPM, but it allows for more controlled results (SCPM's pictures are usually very choppy, and use 5 different tiles to represent one color).