Sometimes I paint with a 'matching' color to one of the colors in the blank. However, I get very good results from using a completely different color on some blanks. For example, acrylics such as 'snow', 'sugar', 'opal', and 'coconut' are all two-white blanks. That is, two different whites where one stays opaque and the other is semi-transparent. Using a rich color (like ruby red, royal blue, etc.) looks great. Even two pens (same kind of kit) using the same blank, look completely different with a different inner-paint job.
BTW, when I paint, I paint both the inside of the blank, and the brass. That way there's a layer of paint on top of the glue which helps hide bubbles in the glue. Of course, having two layers of paint (and sometimes, nail polish) means I occasionally use a slightly oversized drill bit.
Have fun!