On Mac, DigitalColor Meter. Note DigitalColor Meter used to be a reliable way of finding out what RGB values were being sent to the video card.
(Intended as reply to the OP)
In my good old Windows InDesign CS2 it works as follows:
1. Open Window > Colour
2. Press ALT and click by Eyedropper at any position
3. Read RGB numbers
This is valid for sequential measurement. Read helptext "Eyedropper".
Best regards --Gernot Hoffmann