The sketch layer is accessed with the 'eye' button in the bottom-right corner of the canvas screen. Clicking that button toggles visibility of the sketch layer

The sketch layer is accessed with the 'eye' button in the bottom-right corner of the canvas screen. Clicking that button toggles visibility of the sketch layer and reveals three more buttons, and clicking the newly-revealed pencil button toggles drawing to the sketch layer instead of the canvas. The decision to implement only two layers for Cobalt was a conscious one. The design of Cobalt is focused towards speeding up the user and helping them to finish their images, and I found that being able to go back and tweak each layer made it more difficult to commit to a final image.