If you're reviewing your self-launched art before launching it and you notice that the colors don't look how you expected, it usually means that you exported your .jpg in CMYK color mode instead of RGB. Luckily, this is easy to fix! Just export your file again in RGB mode and edit your files. If you've already launched your print when you notice that the colors are off, you can hide your product from your store and then relaunch it.
Note that you should work in whichever color space makes the most sense for your medium (usually CMYK for vector art and RGB for scanned art and photographs) - just make sure to export the final .jpg as an RGB file. Check out this article on Art File Prep for more info on preparing your art for self-launch.