There are a number of options here.
1. Simply find free, open source icons online and then upload those to use in the custom icons section.
2. If you like the current icons, but just want the colors changed, you could download them and edit them in Photoshop (or a free alternative like http://www.gimp.org/) to colorize or color-overlay.
3. You could buy the source icons for editing (http://socialcons.com/).
4. There are different CSS techniques that could work like
filter, though not yet supported in all major browsers.
5. An icon font like Genericons (http://wordpress.org/plugins/genericond/) could probably be utilized.
Based on what you’d like to try we can assist further.