Then I vary how hard and soft the edge is depending on the mark I want to make. If you are using the standard round brush, I usually have my opacity to about 80 and my flow to about 80. Blending: Mess with your opacity and your flow settings on your brush.If you're going to experiment with something, do it on a separate layer. Painting just over the B&W tones will likely get you a sort of soft, blurry, often muddy look that can get ugly really fast if you don't have your color theory down pat! Except then I add another "normal" layer on top and paint in more details and make some changes. This is sort of it, where he adds the red to the hat.
When i use this technique, I do the b&W layer, then add some rough basic colors over top to feel out my color scheme, then take that and add another layer where I am painting with straight, opaque pigment to get it the last 30% of the way to finish. This is an ok way to paint but I've found that it A) can be very limiting and you spend a lot of time searching for the right color, hue, and layer settings, most likely adding more and more layers to try and get it right and B.) doesn't teach you anything about color. Once it looks like a finish B&W piece, then you add a color layer or two on tops, and start painting over it The layer will add pigment without losing your tones. The tonal studt you've done is decent but you needed to keep going with the lighting/volume painting before trying color. But if we used Kyrie0201's style (which feels like airbrushing rather than, say, oils, if we're pretending digital mimics tactile mediums) here's what you need. Ok, so, there are like 1 billion different was to approach digital painting and get what you're asking.