It's not the hair color as much as it is the skin tone of the person. Alba and Jackson have the same skin tone....Benoist and Slater do not. Their skin tone lends itself to lighter hair color, Jackson and Alba's does not. Alba with honey blonde hair was fine, but with platinum blonde hair was just no no. So, Jackson with the hair color of what is in the comics would have looked horrid. Why go that drastic, or even a little lighter if you are only going to be on for a few episodes? So, I'm glad they simply kept her hair as is, rather than go with a blonde wig....that hasn't gone so well on this series. This season is the lightest that Melissa has kept her hair for the entire season. It was much darker in the 1st and 2nd season....this season, they have kept it much lighter.
IMO, if the person is only going to be on for a portion of the season, no need to fry their hair with peroxide.
Also, they did nothing to Alba's skin in the 2nd movie, they used a platinum blonde wig rather than using the honey blonde lightening of her hair of the first movie. Nothing was done to her skin which was why the platinum blonde with her skin tone looked ridiculous. There is no way Alba would have allowed them to bleach her skin....she fought them on her hair which was why they went with the wig for the 2nd movie. She had to cut her hair after the first movie to remove the damage. Melissa has enough red/auburn in her hair that simply lightening it and not bleaching her entire head of hair has kept her hair in good shape. If Jackson colors her hair to get the color she has now, which she may very well do....bleaching it anymore would damage her hair.
So, why take that chance for a few scenes in a few episodes of a TV series, when your real money comes from product advertisement in your home country? She would be stupid to do that....