Can't be collision course, and not heat related either. I went on a small test drive, this time I aimed for a water world. That too changed to red when very close, so maybe it's some sort of proximity alert or the speed thing Roger mentioned. Impact alarm is in the top right corner, and when I pointed the nose away from the planet when it was red on radar, it remained red.
Next time I'll do the test with some other ship than Cutter, the brakes on that thing are not very good.
(Don't worry, no hull damage this time)
I wonder if there is a complete nuts and bolts manual somewhere, the one provided by Frontier itself does not cover everything.