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So I'm a pretty novice painter, I'm working on some Chaos Marines that have a "snake" feel to them, I'm trying to go for a green but the colors I'm using seem a bit too neon for me, so I was wondering if I dry brushed a darker green over a lighter almost neonish green, would it produce a sort slimy green color, like a sort of glow behind the armor?

That's a really hard question to answer, it depends entirely on what paints you are using and over what surface.

I think I know the effect you are after, it won't be easy to get a realistic 'snake' feel. I think your logic however is prob not far off. Just be careful to not overdo it and tint the under colour to take away the lighter colour. (Hope that makes sense).

I'd like to see a pic when you have one painted up.

Try it on something you don't mind experimenting on (not the cat).
An old model or toy car or even some leftover sprue

I'd have thought a dark green wash would look more natural

I think I get what you're asking for, but I'll cover both my bases and you can go from there if you like what I provide.

If you base-coated the model a "neon" green, and you want it to look like their is a glow coming from "behind and between" the armor, paint the armor a darker color (as you suggest), but also take a look at "Object Source Lighting."
I suggest this link here: http://www.dakkadakka.com/wiki/en/Zenithal_Airbrushing%2C_OSL_and_Snow_and_Ice_Tutorial%2C_subject_-_Grey_Knight

Paint the armor darker green, and highlight the edges of the armor to begin matching the lighter green that emanates from between, continuously highlighting the light green up to a white at the very center. This is tricky to get right, so don't be mad if you don't get it on the first try. Takes practice!

If you're looking for an actual "slime" look, I'd take a look at these tutorials instead:
Slime: http://hephsforge.blogspot.com/2010/01/making-tyranid-ichor-ooze-and-slime.html
Stringy gore kinda thing: http://massivevoodoo.blogspot.com/2009/10/tutorial-making-ugly-strings-and-stuff.html
More slime, but not as good as the first link: http://massivevoodoo.blogspot.com/2009/11/tutorial-making-glibbery-slime.html

I haven't had an opportunity to try these options myself, but if you've got a spare model sitting around, do some experimentation and see what kind of trouble you can stir up. Hell, even if this wasn't what you were looking for, I hope it helps you in the future.