What is the white pattern on trim wheel for?
The white marks make it easy to see if the trim wheel is moving, which would be tricky if it was entirely black.

But wouldn't the pilot always know if they was spinning the trim wheel? Remember that the autopilot can also adjust the trim, which might not be obvious to the pilot. The visual marks make it easy for the pilots to see what the autopilot is doing.

Example video (not an Airbus, but very similar):