We merchandise mid-priced pens (under $100) by colors, all the reds together, all the blues together, etc. We wouldn't set up a show without a significant offering of red, but it would be rare that red was the top selling color. We track sales by color from year to year so that we can plan for what to display each year. If one color starts selling well, we pull more of that same color out of the storage cases and beef up the presentation.
School colors also come into play, if we are selling in Bloomington, IN, Columbus, OH, or Louisville, KY; all of which have major universities who use red as their predominate school color, then we will put out a few extra red pens.
Which colors sell best? I think you need to work that out for yourself and the markets in which you sell. There is considerably variability from one show to the next.