Ok so the "official 18% grey paint" is something like $80 per quart and up. Thanks to this tip from Dan Desmet at Flanders Scientific, here's a really REALLY close knock off of that color for a lot less.
You want Sherwin Williams SW 7071 color.
The one to get is Duration Home, Extra White Matte Base, 6403-63925. The BAC code should be
BAC Colorant 02 32 64 128
B1-Black - 20 1 -
Y3-Deep Gold - 5 – 1
Why do you need 18% grey? Well it helps to keep your eyes neutral while you edit and color grade a scene on your monitor. That's the theory anyway.