Next copy the adjusted sketch and paste it (Cmd/Ctrl + C > Cmd/Ctrl + V) into our new document, and resize to fit leaving a little space around the illustration.
Create a new layer and name it ‘Colour 01’. Start to colour up the illustration using the Brush tool (B). Pick a single colour and begin colouring the relevant areas. The colour will sit behind the linework because we’ve set the outline layer to Multiply – this helps us see where and what the design looks like while we colour it up. This design will have four colours. We need to make sure that each new colour is created on a new layer and named for our reference: this will make it easier to split out into colour separations later.
We need to ensure that we slightly overlap adjacent areas of colours to avoid gaps caused by registration errors in the printing process. This is called ‘trapping’.
The best way to add trapping to your colours is by adding a stroke to each colour fill. Do this by hitting Cmd/Ctrl + click on the first colour layer. This selects your first colour. Then choose Edit > Stroke and enter the width of the stroke you want to add, set the colour to match and the location to Outside.
Check whether you need to add more or less stroke by changing the opacity of the outline layer so you can see the colours through the outline. Try and get your colours to sit half in the outline.
Once the design is coloured we can give it depth by adding highlights and shadows. This we do in Illustrator so we can get sharp lines. Save your design so far and open up Illustrator. Choose File > Open and choose your saved design, click open and in the import options choose ‘Flatten Photoshop layers to a single layer’ as we’re only using it as a guide in Illustrator. Lock the design (Cmd/Ctrl + 2) and hide the artboard.