1. New gradient layer corresponding adjustment layer.
2. The gradient selection is built in black to white.
3.ctrl+shift+alt+e stamped a new layer and then ctrl+i to make negative effects.
4. Menu select "Filter ➜ other ➜ minimum", strength 1px.
5. Change the blending mode to highlight the color.
6.ctrl+l to improve order, fine-tuning anchor points to remove some noise, and overall optimization of the line draft. You can then use the highlight/deepen tool to do local processing.