Is your brush set to Normal blend mode? I just duplicated that color disparity by setting mine to Screen.
Thanks for the response. Yes, my brush is set to normal blend mode and the eye dropper tool is still not working. However, I opened a new document and it works so I've narrowed it down to I must have accidently changed a setting in that document specifically. Still new to digital art, so trying to figure it all out. I'm happy to know it's just a specific document though and not the software itself. I'm sure with some fiddling I'll figure it out now, again thanks for your help.
One possibility - you do not show all of your layers.
Your eye dropper tool is set to "All Layers" which means, even though you highlighted a single layer, it will pick up the colour made from the combined layer stack at that point - including any adjustment layers and layers above with blending modes. If you then paint with that colour onto the single layer those adjustments / blending modes will be applied again and the effect will look different.
This can be avoided by using sample current and below
A single filled layer with a curves adjustment layer above to lighten it
Brown patch sampled using "all layers" and painted back onto same layer:
Same brown patch sampled using Current & Below and painted back onto the same layer :
I hope that helps
Thank you, I saw your post just after I discovered that it was my curves adjustment layers that was causing the issue. I've since turned them off and it fixed the issue. Thanks for the helpful explanation!:)