From my discovery, the F30 turn signal bulbs are PY21w. You can pick up halogen white ones but they will still have a slight yellow hue to them, even for the pure white ones. Or you can get the LED bulbs but you'll need to wire in a load resistor to cancel the bulb-out error, at least till someone gains access to code the vehicle.
Here's an example of the white LED turn signal I installed.