This isn't a style similar to my own, so I won't be able to give super good feedback, but here's what I can say: 

The colors are good for the most part. There are a few that are kind of bland (the glow used on the sign, the pants of the duck-men, and the white-suit henchmen. The rest are pretty good for how many shades you have. Even the red looks great on the cyan van, which is a difficult contrast-issue to deal with. 

There are a few different types of lighting that vary depending on what you want the piece to look like. At the moment, you've got arcade-style lighting, which is fairly simple and usually just has an ambient shadow underneath the character so it works decently well with every light source (see duck-men and henchman). Realistic lighting, which I think would work wonders for this piece, has the shadows in the position opposite to where light would be coming from (realistic), so the duck-men would have a shadow behind and below them because of:

1. the glowing sign giving off a red aura 
2. the reflection of the headlights on the wall that bounces back

You'd need to choose a light source for the henchmen in order to change him, but at the moment, it seems to be in the middle of the room, out of the piece's vision. 

The perspective looks wacky, but I don't have experience in that kind of stuff. I think you should go for either isometric OR side-scroller, not a combination of the two, until you're much better (I would never try that, it's so difficult). 

I read a quote somewhere that sounds kind of like "Pixel art is the study of light", because lighting and shadows will dictate everything. If you really, really focus on learning how light works and understanding where you need shadows, what will be dim, what will be bright, you won't need assistance.