November 1 1936 was the "official" date of changeover from gray to red, coinciding with the first day of the 1936 model year. I think crawlers were red earlier than that though.
There is a lot of anecdotal evidence, primarily from old-timers who swear they saw them, that IH delivered plenty of red tractors long before November '36.
I'm with Fawteen below on this one though. It's your tractor, paint it the color you like best. Any busybody who tells you its wrong has no way to prove it.