Were the white or yellow crosses still on German tanks in significant numbers during the Battle of France or had they all been changed to black/white ones by then.

By significant, I don't mean the odd one or two here and there but enough to portray on wargame models without it being totally wrong.

Cheers,

Nigel

There was plenty of time to repaint and repair vehicles after the Polish Campaign. This was the so-called "phony war" period, when the French and Germans just glared at each other for a while, so yes, the crosses were all repainted to the new standard, either black with white outline, or (in a few cases) white outline only (since the base Dunkelgrau was so dark, anyway).
The two-tone gray and brown camouflage scheme was still standard at this time, but the brown was tonally so close to the gray, that it is all but invisible in black and white photos. Some vehicles may have been painted in overall gray, anticipating the official change just after the French Campaign ended, but it's hard to be sure.