vim color settings

I'm trying to define custom colors for vim's syntax highlighting, but I ran into some behavior I can't explain or work around. There's light and dark varieties of all the available colors and I was trying to make the default text white. When when I do that (or set it to any of the bright colors), none of the dark colors are available, they all become bright (gray excepting). Darkgray still exists, but lightgray becomes white. All the other dark colors just brighten to the light bretheren.

An example in case this isn't making any sense: load up the test color file (/usr/local/share/vim/vim61/syntax/colortest.vim on my system) in vim. Run ":source %" and it should colorize itself. Then set the default color to white ":hi Normal ctermfg=white". See all the color changes? Just set Normal to gray and everything's as it was. If I set normal to anything that doesn't have the dark prefix (barring lightgray, black, and brown, which is really darkyellow), I lose all the dark colors. Since I'm working with a black background, the only disabled color I really want is lightgray, but I don't want it as the default.

Is there something I missed? What else is Normal changing when it's bright? TERM is set to xterm-color right now, but playing with it hasn't had any effect. Light or dark background settings hasn't either. I'm running FreeBSD 4.9 with tcsh.