This tutorial is based on this image:
Step 1. Start with an image.
I already clipped the background out of the image so the effect will only apply to the main image.
Go to the Channels Palette and select the channel that isn’t too dark or too bright – I would suggest a channel with medium contrast. Press CTRL (win)/CMD (mac) while clicking on the chosen channel.
This will create a selection. Make sure the image is selected – that is, shading on our main subject is selected and not the background. You can Invert a selection by pressing CTRL(win)/CMD(mac) + SHIFT + I or Go to SELECT>INVERSE.
Step 2. Adding the Colors
While the selection is still on, go to the Layers Palette and Create a New Layer.
Fill the selection with R:0 G:255 B:0.
Create another layer and fill the selection with R:255 G:0 B:255 which is a hot pink.
Step 3. Offsetting the Layers
Deselect the image – Ctrl(win)/CMD(mac) + D.
Move the pink layer 8 arrow key hits to the LEFT. Then move the green layer 8 arrow key hits to the RIGHT.
Step 4. Masking
Select the green layer and click Create Vector Mask at the bottom of the Layers Palette.
Choose the Gradient (G) tool while the vector mask is selected in the layer, swipe it horizontally to the right so the green will show only on the right of the image.
Repeat the same steps above with the Pink layer but when creating a gradient vector mask you swipe the gradient the opposite direction so it will show pink on the left side of our image.
Once done, change the Blending Mode to Overlay on both layers.
Step 5. Refining
You can finish the whole image by using the Eraser (E) Tool or the Brush (B) tool to fine tune (add or delete) parts of the Vector Gradient Mask to show more of the picture.
I would suggest a soft brush (Hardness:0) that is at least 80 pixels in diameter.
