Now using the Pen tool (P), add white highlights to the design. It’s worth taking your time here to follow the shape of the design; play with additional shapes such as circles. Once you’re happy with your highlights, select them all using the Selection tool (V) and copy and paste them into the Photoshop file. Rename the new layer ‘highlights’ and rearrange the layer order so that the layer is directly below the outline layer.
Repeat step 7 to add the shadows but this time using a fill of 20% black. Once you’re happy, select and copy your shadows. Go back into your Photoshop file and create a new layer. Name it ‘shadows’ and change the blending mode to Multiply. Paste your shadows in as pixels and arrange the layer so it’s directly below the outline layer.
The design should have a lot more depth now. Earlier we said it was a four-colour job, but now we’ve added the light grey shadows it’s looking like a five-colour: this is our secret weapon. Now we need to turn the shadow fill into bitmap dots that we will later merge with the black outline layer to create one colour with two tones. Select and copy your shadows again. Choose File > New and create a document in the same size but in greyscale. Paste the shadows onto the artboard.
Now choose Image > Mode > Bitmap, set the output to 300dpi and set the method to Halftone Screen. Now in the halftone screen set the frequency to 45 lines per inch, the angle to 90º and the shape to Round.
You’ll now see that the solid grey fill as been changed into black dots that when printed will give the illusion of a grey tone. If you’re happy with the number of dots Select All and copy it (Cmd/Ctrl + A > Cmd/Ctrl + C).