u can download the RR engines from here (since the default and california livery uses the same colour engine textures)
https://forums.x-plane.org/index.php?/files/file/74800-united-airlines-n48127-for-the-flightfactor-757-200/
then take the RB211 folder into the objects\757 folder of the california livery and it should work if the mod is installed.