Two things happen when light strikes a car. The wax/polish layer reflects light like a partial mirror, allowing most light to pass through to the pigment below. The pigment will either absorb light or scatter it according to its colour.
Assuming both cars are equally glossy then the reflection will be equal in both cases.
The black car absorbs all light that passes through the gloss layer and so the viewer only sees the reflection.
The white car absorbs almost none of the light passing through the gloss layer and will scatter it in all directions. This extra light is bright in relation to the reflection and so the reflection is hardly noticed.