ColorZilla is an extension for Mozilla Firefox and the Mozilla Suite.
With ColorZilla you can get a color reading from any point in your browser and more.
I use this all the time when I'm trying to match colors and it helps me to get the best color combinations for my blog.
For example, you can use it for to match colors of the Google AdSense ads and your website or blog.
Once you have installed ColorZilla on your Firefox, you will find an "eyedropper" icon at the bottom left of your status bar.
Click it and you'll get a crosshair cursor.
Point the cursor at any area or text on your website or blog and you will find the RGB values and hex value (e.g., R:255, G:255, B:255 | #FFFFFF) on the status bar.
Here's the link: