I have a few comments (mainly on the colour and finishing), so hope you don't mind? anyways...
- not sure how you got the white pixelly bits around the edge - erasing, I imagine? erm..... if they weren't there the finish would be much cleaner and more professional looking...
- you can slightly see the brick pattern through the character's eye.
- the shading's a little irregular - in some places it's nicely 3d-looking - the depth is good - in other places it looks very flat. I think the variation of shade tone could be greater (more shade colours and light colours) and the shadows around the edges bigger and getting darker towards the edges - to avoid the flatness effect. He (?) kinda looks like he he's flat with bevelled edges in places.
- the background and foreground colouring styles don't match too well - the background's veyr clean with sharp edges, whereas the foreground looks soft, and they clash a little.
- the hand holding the spray can looks a bit awkward to me - probably the can would be higher up in his hand, so those three fingers didn't look so distended. at the moment the hand looks a slightly unnatural shape.

thassit, for now - hope it helps! looking forward to seeing more of yer stuff soon, Vanilla! =)

otherwise the pose is pretty cool and the colours work alright together - kinda fresh 'n' funky.

And listen to the comments of the others they have some good points.

I'm sure if you learn to avoid some of the beginner mistakes you will do great.

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Wer nicht vom Meister lernt, lernt nie etwas.