Prep the layers

Set the line art layer to Monochrome via the Layer Properties window and then convert all white elements to transparent, by pressing the Black Only button in Layer Properties. Next, drop the Opacity of this layer down to about 60 per cent, which will help ensure that the flats are under the line art rather than running against it. Finally, create a full- colour layer below the line art and select it. This is now your ‘ flats’ layer.