my guess is that in the manga, they wanted her to have purple hair, and when they looked at it in the anime, they decided that blue might be better to go with. that is my opinion. i have been curious about that too.
I have some pages in the manga colored by Toriyama and I can say that there are many colors that do not match the anime, probably the TOEI ANIMATION had decided to change them because with the original colours the anime would have an older look. For example nam have a pink dress in manga :)