I learned while making a thumbnail of a picture. It will then take you to the results page. Here your original picture will be at the top of the page with the details of how many bytes your picture contains. This will also reduce the loading time as well as saving you storage space. I usually start at the 16 color level, but this will depend on your picture quality output.
Some pictures can go with even less colors and still maintain a good quality look.
Click onto the amount of colors you want to try, be sure to experiment with a few before making your final selection. After you click onto your choice of colors, your picture will come up again with the reduced colors. You do what is best for you. If you have any problems or questions E-mailMe and I will reply back to you as soon as possible.