Im guessing the artist went to the channels tab on photoshop (right next to layers) and moved the red/green/blue to different directions, then stamp visible, copy and paste on the final work and delete or lower the opacity until it looks good. that's how I would do it
This is by far the BEST Sailor Mercury fanart I have seen so far...... The lighting, the shading, the colors, the softness of her face, it's all perfect! Congratulations on your very well-deserved Daily Deviation!