Hm. You're right.I guess they do take surface area into consideration. I'm not sure then. I was mostly basing that theory on the UK/France comparison, because I'm pretty sure the UK has less blue in it than France's, but they're like identical colors on this average map.

I think they categorized the colors into "red" "blue" "green" etc, regardless of shade and then counted the colors that appear, and then took them all as appearing an uniform amount, and calculated the average that way. A very quick and napkin-mathy approach.