Sampling a document colour is easy in Photoshop with the Eyedropper Tool. Sampling a colour outside of Photoshop requires an extra step. Details below...
Patreon: / michaelbullo
With the Eyedropper Tool (keyboard shortcut: I) any colour can be sampled from within a Photoshop image. To sample a colour from the Photoshop interface or outside of Photoshop with the Eyedropper Tool, first click within an image or the canvas area within Photoshop. Then, while holding the mouse button down, move the cursor outside Photoshop. Releasing the mouse button will sample whatever colour the cursor is over.