Personally for me, if it was a prestige sports car like a Porsche, Ferrari, Maserati, Lamborghini etc and the car was for me, I would steer right away from cars that the owner has deviated away from the original factory paint code. Missing or incomplete factory service history would also kill the deal for me.
Not worth the risk.
On something like a family hack/truckster, if I bought it for beer money, I wouldn't care if it had been colour changed before, as it's more a matter of utility, than resale value. Having said that, red cars aren't popular amongst locals on everyday cars, as it looks a bit Taxi esq etc. So in your case, it probably wont make any detrimental difference to resale value.