Between the original series and Alien Force, and because the artists thought it looked better.
While Blaziken is correct I kinda like your theory better, only problem is that it's pink in the flashbacks but I guess we can assume that it's because she got really good at using her powers by the time they turned eleven
I ask myself this question when Alien Force first debutted back in 2008 and why the writers would change Gwen's magic from blue to pink. My guess is that because Hex and Charmcaster use magic made from pink mana, they decided to do the same with Gwen.
Weird that Ben never asked Gwen: "How come your powers change from blue to pink?"
Maybe a colour scheme with gwen's clothing, but then her shirt was red and not pink, so then that isn't right either and gwen was wearing a blue shirt in the first half of Alien Force and also wears blue in omniverse, so that is completely wrong. Maybe they just wanted a two - tone colour combination and they didn't like the blue with the blue on gwens shirt...
i Kinda agree, maybe it is because of practice and experience. Well as they say, practice makes perfect!
The most likely cannon reason is because a celestial sapien decided it should be so, an explaination for most all aesthetic alterations throughout Ben 10. However, it is arguable that it was blue before she was tapping into her Anodite mana. Meaning she was tapping into her Anodite mana before she knew she was an Anodite. Or mana color depicts the quality of mana in general.