If I interpret your question correctly, you want to know which 256 colors to pick to a palette to make things look good.
This is called the black magic art of color reduction. There's plenty of ways to perform it, some work better than others, and all of them are wrong, as what you're doing is removing information.
One common way is to place all desired colors into an octree and then reading out the 256 biggest cells; another is to do median-cut of the color space until you have 256 segments.
Easiest solution would probably be to drop all the desired colors into photoshop (or something similar), and convert to 8-bit paletted picture. Photoshop, at least, contains variety of methods to do the conversion.