Judging by color is very subjective, since lighting conditions vary so much. It helps to slowly heat the steel up checking with a magnet as you go, it will gradually become non-magnetic. You will want to heat it a bit past that point and as suggested previously a brief soak at that temperature is helpful. Many file steels are very similar to if not actually 1095, W1 or even rarely W2 so a little soak will help get some of that extra carbon into solution.
One thing I like to do to zero in on the proper color I want to watch for in any given lighting situation is a trick I learned from Don Fogg. Heat a piece of the steel you are using (the blade you are working on is fine if you are doing a normalizing heat) to above non-magnetic and bring it out of the fire and place it in something very dark like a metal bucket held upside down so that you can watch the steel. In still air it will gradually darken and if you are watching closely you will suddenly see it brighten and even out again back to a previous shade where it will hang momentarily before continuing to darken. This point where it 'brightens' again is called recalescense (it happens also when the steel is being heated but is harder to see since it's in the fire, on heating this point is called decalescense) and it marks the phase transformation from pearlite to austenite on heating or austenite to pearlite on cooling. On blade cross sections it will flicker or flow across the blade due to the differing thicknesses. If you quickly pull the steel out into your current lighting conditions when you notice this brightening you will see the color you need to look for for hardening the blade (for those lighting conditions only!)
