From the top menu, select Drawing > Drawing Tools > Dropper.
Press Alt + D.
In the Camera or Drawing view, click on the desired colour.
If you're using another drawing tool such as the Paint tool, you can temporarily hold down the D key and click in your drawing before releasing the key to pick your colour. When you release the key, you will return to your previous tool. You may also want to select the zones painted with the colour currently selected in the Colour view. This can be useful for removing rough lines from a clean drawing.