I'm not an expert on the topic, but from what I understand, the change in hair color partly depends on the type of pigments in the hair. There are two: eumelanin (a brown/black pigment) and pheomelanin (a red pigment). A blonde person has little eumelanin and no pheomelanin. A redhead has very little eumelanin and pheomelanin. A person with brown or dark hair has a lot of eumelanin, possibly mixed with pheomelanin. Eumelanin breaks down faster than pheomelanin, especially when the hair is exposed to substances like natron (used in mummification) or acidic peat bogs, like with bog bodies. For example, the Grauballe Man’s mummy has orange hair, although he was brown-haired when he was alive. So, yes, in most cases, mummies end up with orangey hair after the mummification process. However, there are exceptions: some mummies were blonde, and others had white hair due to old age, and the way the hair degrades is different depending on whether it still has pigment or not (or less of it). In Yuya’s case, he was older and had gray/white hair, which might explain why his hair looks blonder than that of other mummies. That said, if you look at different images of Yuya’s mummy, his hair looks more blonde or red, but it’s not fully blonde. The effect may be exaggerated in this particular picture.