Hi,

When i first played the game many years ago, the Quen sign was blue during the entire game, upgraded or not. I'm currently rediscovering the game in a 2nd playthrough, and the color of Quen switched to red from blue. I was wandering if it was now a visual enhancement for higher Quen intensity?

It has nothing to do with the sign intensity. It has everything to do with obtaining the Summer's Crown ability.