Poll Results

Total votes: 3734

Pick whatever colour strikes your fancy at the time: 1407 (37.7%)
Pick whatever is closest to hand, paying no attention to colour: 1062 (28.4%)
Try for the same colour, but settle for something else if it's not in stock: 468 (12.5%)
Pick from a subset of preferred colours: 406 (10.9%)
Get the same colour each time: 199 (5.3%)
I don't brush my teeth: 167 (4.5%)
Cycle systematically through a set of colours: 25 (0.7%)

Apparently, if some e-mailers are to be believed, some people never change their toothbrushes. Or only do so when their dentist gives them a new brush.