There are a few ways that that could happen:
--There are a few varieties that open one color, and gradually turn another. Some start out white, and turn lavender, while some start out yellow, and turn orange or red.
--If it is a recently purchased plant, many of those have several cuttings per pot, and one cutting may mistakenly be from a different variety.
--Occasionally, a single plant will have a mutation (aka bud sport) on one branch that keeps it from forming the pigments that turn the petals red, exposing the yellow ground. So you wind up with a single plant with some branches red, and some yellow, and occasional flowers part and part.

Some examples appear to have various colors, but if you examine them carefully, they are actually 2 or 3 different plants growing together as one globe and sold as novelty. They look nice for a season.

as they are on the same stem it may the age change as Tug suggested or the sport as also suggested by Tug. lovely when this happens though.

I've never seen one stem with 2 different colors - purple & yellow, etc. It's normally a variation of one color, as some fade with age, or all the blooms follow suit - white fades to purple with age.