I see your concern, but at the same time, it being on a white card is going to throw the look totally off. Just do it, if it looks goofy on the car, just wipe it off with some lacquer thinner before it dries totally and carry on.
On the other hand, there's no way it'll look worse than a huge white paint chip.
EDIT: Also think about the shape of your test blob..that's going to throw off how the color looks in any given light.