Just because the paint may be 18% gray, which is a reflectivity value, does not mean its a neutral. It's highly unlikely that there are equal amounts of red, green and blue so it can't be used for accurate white balance.
For all intents and purposes, there is no practical reason to worry about using 18% gray paint in the studio. If the studio space is small then white works well in terms of keeping the room from feeling small, but it also gives you more return and reflection from the walls. Black gives you no return but makes the room feel very small and dungeon-like.
Using gray in a small space makes a lot of sense because it doesn't make the room feel too small while controlling some of the unwanted return and reflection to the subject area.
I've also done some extensive testing and determined that keeping the ceiling white makes sense. You really don't get any return from the ceiling unless you're pointing your lights at the ceiling and for those times that you do want to create a large, soft hair light or want to bounce off the ceiling, it's there when you need it.
When choosing gray paint for a studio, the most important thing is not the shade of gray, but making sure that there is no blue to cause a color cast. Almost all gray contains blue unless you create a custom color or specify that there shouldn't be any blue.
