The following facts were noted in Wikipedia.

1.In mathematics, the four color theorem, or the four color map theorem, states that given any separation of a plane into contiguous regions, called a map, the regions can be colored using at most four colors so that no two adjacent regions have the same color. Two regions are called adjacent only if they share a border segment, not just a point.

2.Kempe also showed correctly that G can have no vertex of degree 4. As before we remove the vertex v and four-color the remaining vertices. If all four neighbors of v are different colors, say red, green, blue, and yellow in clockwise order, we look for an alternating path of vertices colored red and blue joining the red and blue neighbors. Such a path is called a Kempe chain.

3.Dichromacy is a moderately severe color vision defect in which one of the three basic color mechanisms is absent or not functioning. It is hereditary and sex-linked, affecting predominantly males.[9]

The 4 color map problem is a classic planar geometry problem regarding the smallest number of colors that are needed to complete a map where no 2 colors share contiguous borders. The classically used colors in the map problem are red,blue, green, and yellow. Color blindness with red-green and blue-yellow patients will generally yield to a blue red choice in map design when there only need to be 2 colors. There may be other sublime and diabolical reasons that red and blue were chosen but in the end we should remember that it is usually the men who are color blind. The blue for Northern states is likely historical detritus from the civil war era.