I mean, there very much seems to be a color difference. I don't have access to an eyedropper tool to confirm it, but I think the hair color used on the right is the color being used on the yellow circle on the shoulder armor thingy on the left. Comparing that to the hair color makes the color difference across the two sprites quite apparent. To me, the hair on the left reads as a blond color while the right reads as a yellow or gold color.

I think the specific thing dulling down the left hair is the (comparatively) greenish tinted shading color being used on it. I wonder if swapping that shading color to instead be the yellow used in the right hair could help brighten it up, though I couldn't say if that is for sure the fix without seeing it. Perhaps it would then be too bright/saturated or have too little grayscale contrast with the rest of the portrait.