i have some that have a yellow and red bicolor flower. Sometimes they have mostly red, sometimes mostly yellow. Are you sure the flowere were entirely red the first year. I haven't seen any that change completely from red to yellow. Hope this helps.

There were about ten to twelve solid red cannas in one part of yard and two red in an other part. For several years the ten to twelve were solid red. Two ago years the ten to twelve all turned to bright yellow, the two in another part of the yard stayed red.