Humbrol used to have a great selection, but discontinued most of the WWI colors. My personal next choices are Xtracolor and Polyscale. There is another line that I don't recall the name of currently, that specializes in WWI aircraft colors, Acrylic, and it seems to be Italian or Spanish in manufacture.
Edit-Misterkit is the acrylic WWI line of paints.
F is for FIRE, That burns down the whole town!
U is for URANIUM... BOMBS!
N is for NO SURVIVORS...
- Plankton
LSM