The infographic above shows the most common colour in various national flags, excluding white and black. For example, red is the most common colour in the US flag. If there are two or more equally common colours (as in BE = Belgium or FR = France), the country is given partial credit for both. Similar colours are grouped using k-means clustering in R.
Overall, shades of red seem the most popular, followed by shades of blue. The set of flag image files I analysed wasn’t fantastic, however, and that may have skewed the results slightly.