Reversing colours of "availability" bar

Currently when customers are looking to book an appointment, the availability bar that appears showing the number of appointment available is grey. When a space is filled, a portion of the bar is filled with white. I think people are used to seeing white as available and grey as "booked and not available". My customers have been confused by the grey bar, assuming that the appointment time slot is booked (not reading the text correctly). I think swapping the two colours would greatly enhance to UX. Thanks!