In a recent referendum, Mauritania voted to add red stripes to their yellow-and-green flag. This means Mauritania has joined the vast majority of countries that have decided that red should be one of the colors on their flag. How vast is this majority? This vast.
No other general category of color (green, yellow, white, etc.) comes close to red’s dominance in prevalence among countries’ flags. A lot of nations find important symbolism in the color red, often representing the blood of those that have died for their country, revolution, courage, or valor, among other meanings.
In fact, if you look at the map above, you’d find that among countries with at least one land border, you cannot avoid either the country you’re in or a country you border having red in their flag if you include maroon as a red. If you exclude maroon, you can achieve this in Uruguay or Qatar.
Note that I have colored non-sovereign countries or territories in the map above according to the flag of the sovereign country that owns the land. If one considers the flags of these regions instead, there will be a few additional instances of flags without red, like Curaçao, Tokelau, and Macau.