I have connected my rbp3 to an old PAL/SECAM CRT television and have issues getting colors working. I edited the /boot/config.txt file and set the sdtv_mode=2 flag and tweaked some settings.
As the machine boots I see the rainbow splash image and also some lines from Raspbian booting in colour (also the 4 berries on top). Then the berries disappear, the screens turns black and white the over/underscan settings are gone.
Something is overwriting my settings from the boot/config file and I don't know what, I sincerely thought that raspbian didn't had access to the actual sdtv settings.
I made a short video to show the issue: https://youtu.be/zbzejiEh_-Y