Something doesn't add up. I have annealed brass at 400 Celsius (only for a few seconds, mind you) and it doesn't show a colour change. Are you sure there wasn't some old french fry grease or something in there that may have stained the cases?
Then oven was likely calibrated in deg. F not C. 1 Fdeg = 5/9 Cdeg so much cooler than your annealing.
NormB