From the Twitter page on their brand assets, here are the official Twitter colors that are used on their site. The top bue is the primary color and is "Twitter Blue." The grays are secondary colors and all have a touch of Twitter Blue to match.
The full color set also includes white and black.
If you're planning on using the Twitter colors or logos it's probably worth your time to visit their branding page as it goes into detail on the correct way to use their logs and colors, and also lists the right fonts to use.