Icons
The Bundler Team has put together a few servers that make it easy for you to get icons from a few popular icon fonts. (The code to run the server is available via Github).
You can get icons from Elusive, Fontawesome, Icomoon, Octicons, Open Iconic, and Typicons. That's 2064 different icons available in 16 million colors (giving you about 34 billion options). If that's not enough, the Bundler also gives you easy access to the approxmately 200 system icons installed on everyone's computer. If you want to preview the icons, just look at some of the preview pages one of the icon servers.
Arguments
All the Bundlers can get you icons, and, to get one, you need to supply at the least first two of four arguments: font, name, color, and alter.
font is the lowercased name of the font (use system for system icons). name is the name of the icon, which should be lowercase except for system icons. Use the exact case of the icon name. color should be a hex color (not a named color). alter should be either true or false. System icons come only in their original colors.
color, then it will default to 000000. If you do not specify alter, then it will default to false. One exception: when you specify neither color nor alter, then they default to 000000 and true respectively.
Colors
If it's a hex color, then you can get it. When calling an icon, just call it with any three- or six-digit hex color.
Color Alteration
Flat icons don't work well when the background matches the icon. Since you can never tell what theme a user will have active, you can never know if your icons will be legible. But, the bundler can fix that for you, with magic.
Just set the alter argument to true, and the icon returned will be the color that contrasts with the user's background. Well, it'll be the same "hue" just darker or lighter.
For more information about icons and to see previews, look at the flagship icon server hosted by Dean Jackson.
For particular syntax for icon calls, see the language-specific documentation.