Certified GKH Gardening Expert
I have not heard that white tulips can turn others white, but I have heard that tulips can change to white. There is a virus that causes a tulip to bloom with white streaks. This is called breaking. It can get bad enough to overcome the color of the bloom.
Another possibility is that the white tulips are hardier than the pastel tulips. This is not unusual for tulips. The white tulips may be naturalizing (spreading) while the pastel ones are dying back. This would make it look like the white tulips are changing the color of the pastel ones.