If you want a wide variety, BearToothWoods has a pretty good selection of solid colors. They are in the acrylic blank section (I would link, but, his site often runs a bit slow and its not loading at the moment.) I generally use BTW for solid color blanks, particularly black, white and gray which I use most often, as his prices are some of the best around. If I can't find the solid color I need with BTW, then I tend to use Beaufort Ink as linked above.
All that said, I have had a lot of projects I've wanted to do that I put on hold, as I just couldn't find a wide enough variety of solid-color blanks in general, even across all the various sellers. Usually they are bright solids...prime red, green and blue, bright solid yellow, pink, orange, etc. I have a lot of ideas for pens that would need other colors, so I decided I would make my own solid colors. I spent some time (even asked a few questions here) and ended up finding Divine Pigments:
A divine collection of opaque, solid color casting pigments!
divinepensplus.com
These include some of the colors I wanted that I couldn't find in ready-made solid color blanks. I also figured since they are simple liquid pigments, I could do some experimentation, figure out formulations (i.e. ratios) to mix other colors I wanted, and should be able to mix as many colors as I wanted. These are nice, flat solid color pigments as well. The only outstanding question I had was: How much would be necessary to make a nice, thick, OPAQUE solid color blank... The opaqueness is a key question, and it might become prohibitively expensive if it requires a lot of dye per blank.
I've had to deal with some other things lately, and ended up switching to turning other things, so these projects are all still on hold, and I haven't done enough experimentation to know for sure just how much pigment is really required. I also wondered if the solid white colored Alumilite resin might help with opacity, even though I'm quite sure it will affect the mix as well (i.e. white base + pigment may well require MORE pigment?)