Hello! Here is sci-fi gun I created. I am doubtful of that blue color, but I don’t know how to improve it.
Any critique is appreciated.
Looks more like a toy water gun, made out of cheap plastic. The shape looks interesting, but there are too many sharp edges, not enough bevels to make accented highlights, and of course, not enough light to create those highlights.
Also the general ergonomics - it would be extremely uncomfortable to hold that grip, with its flat sharp back.
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… but, on the other hand, “I’m entirely comfortable with ‘Steampunk,’ and other things that aren’t really meant to be taken quite seriously.” 
And so, that’s how I interpret this model, and I enjoy it for what I choose to see as whimsy. (And if the OP wasn’t thinking that way … oh well.)
If you choose to interpret this model the way I do, it’s very well done.
The form is eye catching. It looks like your modeling skills are coming along nicely.
I’d worry less about colors right now and more about proportions and lighting. As @Stan_Pancakes mentioned, try to make the trigger more realistically “hold-able” as it looks awkward right now. Maybe look up some reference. The main “sin” you’re committing right now is a black gun on a black background. Maybe try a blue background 
Or you could try some rim lights.
Materials will come eventually. Nice work.