RedGard and wall tile
There is no problem with thinset adhering to Redgard painted backer. Just make sure there isn't a vapor barrier installed behind the backer board or you'll trap in the moisture. Hopefully, you taped all the seams on the backer board. Use a fortified thinset. Let it set 5 to 10 minutes before applying. Set the tile and wait 24 hours before grouting. Good luck.