The reasoning is apparently even more stupid than that. Television networks used to swap between red/blue to represent the incumbent and challenging party during presidential elections. It just so happened that Republicans were red in the 2000 election as the challenging party and then red again in 2004 as the incumbent party and essentially attached the color the Republican party because the 2000 election drug on for so long that it cemented the color scheme in people's brains. Here's an old article from 2004 that discusses it more:
https://washingtonmonthly.com/2004/11/14/red-states-and-blue-states-explained/
It's almost as stupid as how "right" and "left" were assigned to different political ideologies which goes back to the French Revolution and was based on which side of an assembly two different factions sat on. Millions of people bitch about the "far-right" only because some monarchists seated themselves on the right side of a hall over 200 years ago.