hello. so i'm playing around with normal maps for a lot more common things in my work flow. there are lots of tools available to create them, some processes quite elaborate and some quite simple

following the simple route, i did this for split-face cmu just recently:

1. created a tiles map in 3ds max, the applied noise maps and cellular maps to create a varied diffuse color as well as a bump map
2. i applied to a plane, then rendered just that plain to get a black/white image
3. i used an online normal map maker tool to create a normal map
4. loaded that up in my normal bump and voila, a bumpy surface

so i was looking at it at various angles and various light angles and finally notice as very extreme angles, the grout for the cmu i'm trying to simulate, looked like it protruded OUT. in the image i rendered, i set the grout to be very dark - which i thought was "low" whereas medium and light are "high".

then i checked a box that said "flip U" and gee, the bumpy cmu looked better.

so what's the deal with settings to "flip" U V or swap green/red?

it depends on how you make the normal map? also depends on which program you're using?