Choose the right size eyedropper
Out of the box Photoshop comes with eyedropper set to point sample. It is not recommended to use this when working with colors in images. The eyedropper measures the color in only one pixel when set at point sample. There will always be variations in neighboring pixels. To get a color closer to how we perceive it, we use an average of 9 pixels.
The eyedropper drop down menu.
This is easy to correct. In the tools floating menu choose the eyedropper (circle at bottom right). Then in the context sensitive floating menu at the top click on the drop down menu and chose "3 by 3 Average". For a image with a lot of "Noise" use an even bigger eyedropper.
Point Sample
3 by 3 Average
The two pixel "images" ( above ) is taken from the inner circle in the middle of the image ( right ). They are magnified 1300%.
