I am a firm believer of buy the matching solid cardstock when you purchase the pattern paper.
My must haves extra are white, various shades of offwhite/tan/beige, a nice red, navy blue, and black.
What to avoid in large amounts...bright colors like the primary colors. These do end up being the "go to colors" for Holiday projects, but other than that there isn't a lot of use for bright colors and they never seem to match anything else.
and, while I have bought them myself, stacks or bundles of assorted colors do make sense if you think you will use the majority of the colors. But often only a few sheets seem to work out and the rest end up being these off colors that don't match anything.
I am a bazzill girl. I HATE the white core cardstocks.