Found a workaround via ADB, tested on FP4:
use this command to change the accent color, in this example to purple:
adb shell cmd overlay enable com.android.theme.color.purple
If you want to change the color back, use this command:
adb shell cmd overlay disable com.android.theme.color.purple
This also worked for my FP4 for green and ocean and should work for all of these colors (case sensitive):
blue (default)
black
green
ocean
space
orchid
purple