Blue = Mike and yellow = will this is an established character detail in the show (emphasized most often using their clothes OR their bedroom walls), this matters because of the "blue meets yellow in the west" byler theory, in which the Russian code from s3 was a hint that when Mike and Will meet in California they'll start to admit what they have together In s4 we got this borderline confirmed by the way they used the colors in rink o mania and put Mike wearing yellow (wills color) and will wearing blue (Mikes)
