Does it have anything to do with the way the light hits her? One of my guys would always appear this beautiful aqua blue color in the back of the tank, but in the front, he looked quite navy in color. Made him look like a different fish
They can change colour depending on their mood too, I find mine become paler when they are not too happy usually during a water change, when they are relaxed they become very vibrant, other times they may change colour is when displaying with other females, they are bold and vibrant when they are in charge and pale and submissive when not! usually the girls with more confidence keep their colours more, the ones with less either stay pale a lot of the time or it comes and goes are there other fish in the tank with her? if so what types? bettas are quite shy creatures and appreciate hidey holes such as coco caves and plenty of plant cover rather than open spaces, when they feel secure this often brings their colours out a lot more too, females are also more prone to paling up than males, if a male pales up he is either very stressed or sick, not a lot can make them lose their colours the big show offs!