Welcome to PixelJoint!
That's an interesting concept: what's inside this can? It looks like nukage from the original DooM 
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Your piece actually uses 30 colors, some of which redundant, wouterpleizier.nl/pj/index.php. Some places contain unique colors that are only used there. I highly recommend refining your color palette (or using a predefined palette as practice).
You got some "banding" going on in the top and bottom part. There is "hugging" in the green nukage/contents. It's a bit complicated to explain here, but you can read about it on the Pixel Joint forum:
, i really would appreciate some reviews to mek a better work the next time 
, i hope you enjoyed it.
Oh, thanks for the correction