The parties used to take turns with the colors.
I’ve heard that it was in 2000 when the election results were “indeterminate” for some weeks that the public got used to Gore being blue (it was the Democrats turn to be blue) and the public got used to Bush being red (Republican turn to be red that year).
But I believe that as the Democrats became more and more obviously in favor of international socialism, the decision was made that it would be AWKWARD for the communists be be in-your-face RED. So they froze the colors with the patriotic party being red and the anti-Americans being blue.
Of course, the GOPe became just as “red” as the Democrats. So we had two red parties — one wearing red and one wearing blue.
And then there’s Trump.