Cherry red has done the job for me for over 50 years--without any problems.
Amaranth red is ~1200 oF, cherry red is ~1500 oF, and red paint that has been in the desert sun too long (i.e. orange) is ~~1700 oF, so it's easily possible by eye to discern temperature difference of only 100 oF or so.
You don't have to get the entire head gasket glowing cherry red simultaneously to accomplish the task. Just get a region a few sq. inches in size up to cherry red and then slowly advance the torch to "push" that red region around the entire gasket (with the back edge of that region cooling off as the torch advances). Although you will do the entire gasket, at any one time only a few sq. inches will be glowing.
I've never timed it, but the heat capacity of the copper does a pretty good job of timing the annealing for you. That is, if you try to advance the torch too quickly the leading edge won't have time to come up to cherry red so that will force you to slow down. It only requires being at cherry red for a few seconds to accomplish the task. Longer is fine (although, with increased oxidation of the surface), but hotter is risky.
It's safer if you do this with a propane torch since it is possible to melt the thin copper with oxy-acetylene. If you do use acetylene I'd recommend using a "rosebud" tip, although any tip will work as long as you pay attention and don't let any portion ever get hotter than cherry red.
Yes--I use a propane torch and progressively "cover" the whole of the gasket. When all done I ease the gasket into a bucket of cold water. That gets rid of most of the crud and the gasket is then very soft.