The reason you are seeing white and not any other colour is because your white is strongly pigmented. In fact, you will have just as much pink in there as well.
Sometimes this is a case of working too wet with your product as it sticks to your brush though this is not always the case.
Don't worry too much about it unless your product starts to feel gunky or you start getting little presents left behind in your brush from the Polymer Fairy.
One thing you can do is also wipe any residue off of your brush onto a lint free gauze pad before you go back in for another bead.
Hope this helps.