Thanks, that makes sense if that's the reason.
Like I said, for this show I did remember noticing brightness drop a little during Levi's big glorious chase coming up in ep 2, but I wasn't sure if it was due to the time of day progression with the sun setting since all the colors also got deeper too. But this makes more sense since CorbeauKarasu's probably right in that it happens in a lot of anime action scenes as I have noticed it from time to time.
I'm surprised I didn't really notice anything like that during the big fight in the crystal cave toward the end of the season.