One color should show up in your Nook's eventually. The other seven can be obtained via Redd, but he almost never comes, so I wouldn't count on that. If you don't really care about the color, you just have to be patient. If you do care, there's a fair chance you're going to have to trade with others for it.
(Also, if you need to catalog all of the simple panels colors, let me know. I can do it for 30 TBT.)