Females can turn yellow when mouthbrooding. Additionally, a dominant female will occasionally turn yellow in the absence of any males. Unlike many marine fish, however, the female does not undergo a physical change to actually become a functional male, and if a male is added to the tank the dominant female will revert to the normal blue coloration. The final option is that it's a male without any eggspots. What is the composition of your tank as far as the species in it, how many of each, and the sex ratios for each species if you can determine that?