In Aseprite, you can select the Eyedropper tool (regular keyboard shortcut: I) by holding down the Alt/Options key on your keyboard.
While your pencil tool is selected (B), do the following:
- Hold down Alt/Options to quickly switch to the Eyedropper and click on the color you want to grab.
- Let go of Alt/Options to immediately switch back to your pencil.
- Start drawing!
Here’s a quick video demonstration from my YouTube channel: