Thanks, Steve; that makes sense.
I see that Kevin Cashen's website recommends forging O1 at 1800°F/1950°F at the top end and not forging any cooler than 1550°F. Granted that lighting conditions etc. can affect color perception, that would be roughly from a dark yellow down to about a bright red. If memory serves, I started running into trouble when forging the tang section at about a light yellow.
So, I guess if I take the steel out of the forge and see that it's too bright, I should just let it cool off a bit before I start to hammer?