A color picker tool is great if you’re browsing a website and find a color you like in particular. Especially if you find a site that provides design inspiration.
Whether using Firefox or Chrome, The popular ColorZilla might be your first go to. However, as of version 0.5.4, you might find it doesn’t work with Retina display in Google Chrome.
Alternative to Chrome users, try ColorPick Eyedropper instead. I prefer the pixel magnifier, which really zooms in and helps you see exactly what you are picking.
It seems the inverse is true for Firefox users with Retina display. ColorPick Eyedropper is in beta mode on Firefox, and doesn’t seem to work. However Colorzilla works out ok.