The 15 was for normal non-VGASPEC graphics sets, and to be fair, I'm not 100% sure whether it would work properly for terrain graphics (ie. the "fixed" colors may be treated as non-terrain pixels, I'm not really sure). The VGASPEC format represents each pixel only with 3 bits, leading to the 7-color limit. There are no restrictions on the individual colors though--in fact the 7 colors defined in the VGASPEC file replaces the ones in the normal graphics set being used, which is why even objects like exits have their colors affected.
I actually don't even know Lemmix has a VGASPEC generating feature so I can't really speak to that. I know a long while ago I released a "myvgaspec" program that converts compliant BMPs to VGASPECs, and I'm pretty sure the error messages and instructions in my program are pretty verbose.