Base color has everything to do with the end effect. I have a bunch of colorshift powders. You can mix them with a transparent base, clear top coat, or brush them on. As for where you get them, there are a ton of different vendors out there. I recently purchased a bunch off of an etsy store, amazon, ebay, dipyourcar, custompaintingpearls are all viable options. In my experience, you get the most dramatic effects over a black base but some of the lighter powders also a do a great job adding a subtle shift when over white, white pearl or other lighter shades. You just have to do some experimenting to find what you want.