I'm assuming you're trying to add to a seasonal (Christmas) layout?
Otherwise, painting the white pieces to match the gray might be the easier way to go.
Alternatively, if your layout expansion plans include areas with hidden tracks...e.g., tunnels...it would be an opportunity to 'bury' the white pieces.
All in all, masking the rails and using any good quality rattle-can white paint...and maybe more than one coat to suppress the gray...should do the trick. To achieve a more uniform appearance, you might also give a shot of white paint to the white pieces in order to have a compatible sheen.
Sounds like a lot of work. As rare as the white ballasted Fastrack pieces are, I wonder how many other folks have gone this route??
Let us know how it all turns out!
KD