The FAQ has several common reasons why a certain color overtakes. I have three extremely vigorous growers that try to do the same thing. A white I thought I had eradicated at least 5 years ago is in bloom!

Some vigorous growers can root from a piece of rhizome no bigger than the end of your little finger. Never the one you want!

If you dont like the yellow ones, then trace them down to the rhizome and make sure you get the right clump and dig them out and toss them.
I have a clump that I noticed this morning, one side is going to be white and the other is going to be a dark one. I replanted this bed 3 years ago and was really careful when replanting. Stuff happens..more than likely a small piece of something got left behind so I will let them bloom and see what is the right one and move the rest