The expression is very good. The legs seem a bit on the skinny side, but the rest of her is proportional. The guitar seems oblong, and uneven. The background seems a bit off,like she's specially posing for a picture.
Ah, this is just so cute and cool! I agree with the comment below- leaving the character colourless really suits the patterns and colour of the background, and gives the picture a nice effect. I like that you added those words and characters behind the characters too! ( :