I was doing some riddles with my daughter today and mentioned some nations. She had to 'calculate' how many of those nations' flags had the color red in them.
As it happened, all of them did. In addition, a lot of them contain white (which I guess makes sense since the color of cloth is white) and blue.
However, red seems to be the color that is most used and in this Wikipedia article it seems to be confirmed.
This article on Wikipedia shows that 30% of the surface on all flags is red, with white being a distant second with 18%.