Tried it this morning; sadly screen went pink after ~30 minutes of TV programme. This is my .config/autostart.sh:
echo 'rgb,8bit' > /sys/class/amhdmitx/amhdmitx0/attr
echo 1 > /sys/class/amhdmitx/amhdmitx0/output_rgb
To be sure I also issued the commands manually and waited for the pink screen to appear again.