If you buy Schmetz needles, you can download their "color card" that tells you how each needle is colored for its usage.
Here's their website:
http://www.schmetzneedles.com/
Go about 3/4 down the page.
I'm going to try to attach a copy here.
Good question, as I didn't really know the answer myself, until I found this!
**Edited to try to put image in here too**