the colors seem to match her eyes more than anything else...which is kinda cool... but my main issue is the girls hair has giant jagged edges were the hair flows into the background... blur is your friend.
as for opacity issues, you can see were some stuff got discolored.
when this happens to me I do one of two things:
1, alter the opacity in sections, the spots where the discolorations happens can be made more solid (also photoshop lets you alter the fill of the layer. you can sometimes use that instead of opacity, giving different effects)
2, alter the background. offen these discolorations occur from certain anomalies in the background, and you can get rid of them by taking a color that works, and replacing it with the one that does not work.
or the third thing is just leave it... some spots don't look all that bad, and with editing I always say that you are never finished, you are just close enough. (you can always find something to make a little better, but you can ask yourself if you need to or not... if it looks good and you like it... you made it close enough)