I recently booked a flight on Northwest Airlines. They don’t fly a lot of routes that interest me, so I can’t remember ever having flown with them. Here is their seat selector. Which seats do you think are available?
Did you guess the blue seats are available? Incorrect! Those are premium seats available for a surcharge.
Next you’d probably guess the dark seats are the ones that are available to select, since they have the highest contrast. You are, of course, again incorrect.
They’ve chosen white to indicate the available seats, thereby making the process as unituitive as they could manage.