I have some of a 'christmas' cactus that is at least 50 yrs old that always used to flower at 'easter' in vivid pink coloured blooms - like fuschias. However for several years now the flowers have come after xmas usually in jan or feb. It is quite neglected but this years flowers have come through in a 'yellow' colour with a vivid pink stamen!
Certified GKH Gardening Expert
Light and temperature can sometimes cause a Christmas cactus to change color, though normally that is from a less dominate color, like white, to a more dominate color, like pink. It can also happen if you get a genetic mutation, also known as a sport but that would occur on only a single stem.
It sounds like yours, since it was blooming one color and is now blooming another, something has changed with the light and temperature and this is causing the flowers to change as well.