Soil, temps, amount of sun can sometimes affect the color intensity but what also may have happened is that there were other asiatic bulbs (immature ones) that were mixed in with your colored ones and the colored ones have produced babies that aren't bloom age yet and have stopped blooming themselves and the white "extras" are now mature enough to bloom.

I'm glad someone finally brought this problem up. I planted Lily Cassandra,Oriental. I think its a hybrid. It should be white with a yellow stripe down the middle of each petal. This year its a very pale yellow, very pretty but not what it should be. Has it cross-polinated or what.
Jenny has a good answer. Maybe lillies should be planted farther apart.Would love someones thoughts.
Nancy

Hi I visited a garden center yeaterday and I saw the one you are talking about very beatiful. For some funny reason this happen often my sister planted some colors lillium and 2 years later they all turn white. Right now I have 4 colors and I have taken pictures of them and have them mark as to where they are but this is my second year with them so far so good I just hope they don't change color Anna Lisa