Pet Air Freshener on Test Centre : UO Discussion

Ok you know on the Test Centres when you say "give air" and you get your pet air freshener? Is there a way to know when you can get the good colours, like glacial, blaze etc.?

It may seem a silly question but I am noting how from (eg) 0:15 to 0:20 past the hour you get all one colour, then from 0:21 to 0:25 you get all another colour. It's consistent and I was curious. Is it my imagination?