In addition, the MRGBZGrabber, with Shaded turned off in the tool modifiers, will capture just the flat color without any material information. This would be useful if you want to capture an area of the project, rather than just one pixol’s values.
For simple color and material matching, though, the Picker is your best friend! The manual is an invaluable bit of reading for this awesome tool, which can be used to capture information from other layers, and can be used to pick depth, draw angles, and a few other things.
And unlike Photoshop’s eyedropper, it can be used to continually resample the image – a very useful feature!