Color picker accuracy

One other variable that might be affecting your comparisons is display brightness, since the brighter the display the greater the potential gamut, if display brightness is less than needed to match sRGB or becomes greater than needed (not sure who sets the manufacturer's tolerance) that variable will change the display gamut and thus make your measurements less accurate than if the brightness was at it's calibrated level. 

One caveat on IOS that is not in Android is the use of Night Shift technology with adjusts the displayed values to remove the metemeric environmental lighting effects associated with color constancy.  Turn that off in Settings and that may level the playing field a bit more for you.  

Heads up on round tripping color (source profile - capture profile - back to source profile) as noted in some of the suggestions. 

This will be closer than not round tripping but there are rounding errors so Asterix pricing applies ( your results may vary - substantial penalty for early withdrawal, some side effects may occur- etc.)   And these depend on the accuracy of the monitor profile.  So do make sure you have a good matrix based ICC version 4 profile installed and use a good toolset to maintain it or you will not get close using this method at all.  

So the real question your asking isn't how accurate the color picker is, but how much the color actually changes going from sRGB to display profile and still produces an appearance match.....

As Dave said in post #5:

Assign the monitor profile, then convert to sRGB.

That's the proper way to deal with screenshots.

Both grabs generate authentic sRGB right? I'm always in sRGB color mode in PS

Not necessarily. On OSX the capture gets assigned the system's monitor profile.

So set your info in the window's lower left corner to Document Profile. The color picker picks using the front document's assigned profile, or the Color Settings' Working Space if there's no assignment or no document is open.

If the capture opens with a profile assignment that is not sRGB, convert to sRGB before sampling color. If it opens with no profile assignment, I'm not sure what you would do. Maybe assume it is the system's monitor profile and assign that followed by a conversion to sRGB?

When you capture images from phones and even web pages they are only approximately in the sRGB color space so if you are not getting accurate Hex values it's not surprising. There are a few things you could try , as mentioned 'point sample' or you'll get an averaged value. Be aware Hex values change in different color spaces-I think I'm right in saying, but Dag will correct me if I'm wrong. Also try this: Set Photoshop up like this with 'Web Color Sliders' showing.

Put a web page side by side with Photoshop on the screen on the same monitor. Choose the 'Eyedropper Tool' and hold down the mouse button, drag the mouse across to the adjacent browser window and you'll see Photoshop is sampling colors in the browser window in Hex. You can in fact sample any screen color from within Photoshop. If you look at the code for the webpage it should agree with the value in the Photoshop color palette for simple things like text and banners but there will be variations in continuous tone images. Afraid I can't give you a way of doing this for a phone.

Terri

Its tricky with the color picker,  because as like Scooter said,  there's tiny variations, especially in lower resolution images. 

Keep in mind that a solid color in a photo or graphic is going to have tiny variations that aren't obvious to the naked eye. ( Noise and compression artifacts ) Depends on what you are working with. Sometimes the clone stamp is a better option...