Please put sample code so we can test by extra quickly by simple copy pasting.
IMHO, you just forgot the |0x1000000
This works as expected
Code:
reaper.SetProjectMarker3(0,1,false,0,0,"test", reaper.ColorToNative( 255, 0, 0 )|0x1000000 )
or
Code:
reaper.SetProjectMarker3(0,1,false,0,0,"test", reaper.ColorToNative( 255, 0, 0 )| 1<<24 )
https://github.com/ReaTeam/Doc/blob/...ions.md#colors