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 日本漫画 colored 由 Toriyama and I can say that there are many 颜色 that do not match the anime, probably the TOEI 动画片 had decided to change them because with the original colours the 日本动漫 would have an older look. For example nam have a 粉, 粉色 dress in 日本漫画 :)