Next, create another layer, at the top of your layer stack, and call it ‘Grey foreground’ at the top. Set the foreground colour to #969798 and make similar strokes to the ones you just did with the colour. Make sure to set the layer's blending mode to Multiply.
We’re nearly there. Create another new layer and call it ‘Dark grey’ and change your foreground colour to #686868. Set the ‘Dark grey’ layer’s blending mode to Multiply and paint some more shapes and marks using the brushes you created earlier.
Open lighting.psd from the project files, drag it to the top of the layer stack and set the layer blending mode to Screen. Name it ‘Lighting’. I’ve chosen a pink colour, but if you want change it, go to Image > Adjustments > Hue/Saturation and have a play with the values.