letsey_x - this is the best tutorial you'll ever read. You have to read it about 20 times to get the full impact, though. It has never failed to inspire me.
schmiss - here and here
I don't like these icons particularly, and the tutorials are a bit all of the place, but schmiss is someone you definitely want to learn from when it comes to colouring/texturing.
meivocis - is brilliant and uses textures in a way previously unknown to mankind. I'm not entirely sure how realiable these tutorials are, but if nothing else they'll make you think differently about iconing.
oh, and I can't believe this but I just noticed she relocated to infectedframe, I think I prefer the icons over at iconfilth though. I'm not sure... perhaps I just need to re-adjust.
nightcomes - is just rediculous with close-cropping and skintone colouring. In the tutorial here, I think the blacks are a bit too black, but... whatever. :D
I was (and still am) completely in love with this old orangey colouring style. It had a tendancy to get too bright, though. I actually forgot about this tutorial, so I can't wait to check it out for myself :D
neversleeps - works wonders with colouring, light splodges and text and these are a general idea of how she does it :D
tihana - does absolutely absurd things with colouring. Just ridiculous. She posted a .psd here which I actually have yet to look at myself (although it's bound to be mind-boggling)
drankmywar - uses saturation and contrast like nobodys business. I don't particularly like the icon this is a tutorial for, but it gives you some interesting insight into what's going on in that brilliant noggin' of hers
mouthfullofdust has been making absolutely amazing icons for yonks and yonks. The colours in her icons are so subtle and perfect. I can't find any recent tutorials, but there are some old ones here and here
and finally, my colouring/iconing tips:
DISCLAIMER: 99% of this was stolen from other people and you'll find it in the tutorials above, but I think is worth repeating because they're tips that I actually use
-don't be afraid to use masks. If one effect makes one part of the piece look awesome but another terrible, just mask away the terrible.
-a high-passed layer with a large radius set on soft light makes for a good substitute to a regular layer on soft light and makes light splodges less noticeable and intense
-make your art big, blend it onto a new layer and then resize it down so that it doesn't end up being grainy or oversharpened
-don't worry if your text is readable, just use it to emphasize the colourscheme or mood of the piece
-use a brightness/contrast layer at the end of the piece to make it brighter and more contrasty, because photoshop makes stuff look brighter than it is for some reason...
-taking away the reds from an icon can make it look surprisingly classier. Just use selectice colouring to add cyan, or subtract magenta and and a bit of yellow. Be sure to compensate and add reds into the yellows. And while you're there add a little white to your blacks, just to make them less intense. Obviously you don't HAVE to do either of these things, but I find it makes icons feel classier.
texture-finding places: (for a couple of these you'll have to backtrack a bit to find the good stuff, but there is most definitely awesomeness in all of these) ... (probably :P)
freshmakers is a better version of texturise but sometimes awesome texturemakers fall through the cracks so I watch both just in case.
or hey, why not check out my textures?