That is odd. Years of selective breeding was required to create shrimp that would breed true vivid color mutations, now common among captive bread shrimp.
IMO, these are two most probable causes,
1. Shrimp in your tank were interbred with a yellow variant, at some point.
2. There is something wrong with the shrimp.
It seems to me that 2 is much more likely. If your shrimp did have the genetic predisposition for yellow, you would have seen a steady increase in the number of yellowish shrimp, and drab shrimp with little to no color. When two color variations of the same species are mixed, they tend to produce shrimp that have less color, like 'wild' shrimp. As colonies like this continue, each following generation will tend to dilute to a certain point. They don't truly revert to wild coloration, but they will never breed original true color. Among my friends and I, we refer to these as 'mutt' shrimp. It is for this reason that I never keep shrimp that even have the possibility of interbreeding, in the same tank.
If it were my tank, my first reaction would be to check for the buildup of toxins in the water. I would also quarantine the affected shrimp. When an adult shrimp changes color, it is usually a sign of stress/illness. Do the yellow shrimp glow faintly in the dark? Sudden glowing (bioluminescence), is a sign of a bacterial infection among shrimp. The infecting bacteria will slowly kill the shrimp that glow, and can spread the bacteria.
I think that quarantine of the yellow shrimp is must. Even if they turn out to be healthy. By doing so, you can help preserve the 'good' red color genes, and possibly create a yellow colony.
Good luck!
