No way! What's frowned upon is useless, thoughtless bgs. Even a white bg is okay if it has a purpose. No prob here =)

Doesn't look like 18 colors. Try to reduce that to 8! (would look better + great practice). Agree about the glasses, plus they don't need an outline.

Single color background? What do I do about it? Transparency? 

Thank you for checking this out though. I am just starting out and this really helps. Here's the inspiration -2.bp.blogspot.com/_RJW0G86N5xo/TGk4tQetdZI/AAAAAAAAMtQ/qDFyrabkkAQ/s1600/IMG_6700+3.jpg

Firstly, the glasses seem to stick out the side oddly, making his head look a little bit lopsided, and secondly, you aren't supposed to submit peices with a single colour background. Other than that, looks great! What's the inspiration material?