Eyedropper lets you pick a color from your screen, and see it in various formats without any extra fuss. When you want to extract a color from your display, just click on the relevant pixel and find it's exact shade. The app holds a list of your previously picked colors, and even lets you see a selection of palettes generated from the one currently selected!
You can follow the app's creator here: @FineFindus
A screenshot of Eyedropper, showing the color #3584E4 picked
A screenshot of Eyedropper, showing the color #9141AC picked, and several other colors in the picked history
A screenshot of Eyedropper, showing various palettes generated from the color #9141AC