Use the Pick Color tool to choose any new color that contrasts with the main color of your chest. Then click the Add icon to be able to add voxels one at a time to the project.
Tip: Undo and Redo
The arrows at the top center of the screen will let you undo your work back to how it was before you made a change, or put those changes back if you have already undid them. You can also use the keyboard shortcut Ctrl+Z or Cmd+Z to undo.