>>132657
I made the pic you’re asking about
I like to color the whole photo using the channel flipping method (turn the blue channel red and red channel blue, then lower the curve, increase the shadows/highlights, add 3-5 to the hue and (usually) decrease the saturation.
Then I add a black mask to everything and use a white brush to turn what I want blue. I set the edge of the brush to soft and the opacity to like 10%, and go over everything I want. Since the opacity is so low, most things are a pale/faded blue, and then I go over other parts repeatedly to make them darker (like the tip of the nose). If you go over one part 10 times with opacity 10%, you get 100% through the mask.
Using this technique you get a lot of control over what’s blue and what’s not, and you get that nice fade. Feel free to ask any follow up questions if anything isn’t clear