Not quiet sure what your describing? Do you have 2 outside lights, one white and one yellow.
Or
Do you have 1 light fixture and it changes from yellow to white?
My outside lights take the standard old style automotive bulbs, 1156, which is a single post bulb with one filament (used for running lights). The 1157 is a double element post. One filament is used for running lights, the other for turn signal and stop lights. The taller one is the turn signal-stop light.
However, if one light fixture fixture turns from white to yellow, I have no clue what that kind of bulb would be:
1157 (double filament)
1156 (single filament)
The 1156 also comes in Amber color too: