Hi Matheus, nice illustration you've got here!
Colour: I think your colours are quite okay, since I can clearly distinguish the character. Though it might be more interesting to go for a more orange colour, since that is the complimentary colour to blue.
Background: I like the magical symbols and that you haven't been lazy about it. But it seems to be directly touching her, almost swallowing her, where as it should be separate and far away. It almost looks like she's in a stomach or something (also because of the colour). Try making the character darker in contrast with clearer outlines.
Character: I'd say her face is a bit too wide and her eyes too large and wide - this is off putting for me. I do really like her expression.
Good luck 
