I'm seeking for a specific way to use the Brush Tool so that the result be from zero (light color) to dark color. From 0 to 100% color. I want paint hairs but full dark color is a bad idea.
After choosing a brush, a size, a top and background color, adjust the brush Dynamics: Shape Dynamics and Color Dynamics.
Shape Dynamics > Control > Fade. The quantity depends on you and the brush size
Color Dynamics > Control > Fade. It goes from top to background color, the same number than the fading on Shape Dynamics.
If you need the brush going to transparent, you should paint over a layer mask with the same brush from black to white. The images below shows the positive and negative version.
This is from black to white over a white mask:
And this is from white to black over a black mask:
Those are two samples of using this brush for masking:
Without a drawing/graphic tablet the best you can do in terms of opacity dynamics is to utilize the Fade option in the Brush Settings.
Transfer > Control to Fade then the text field to the right of Fade controls the drop off to transparent. Higher numbers mean longer strokes before the brush is transparent.
It always start at 100% opacity and then fades to 0% opacity. If you did have a drawing/graphic tablet, you could use the Pen Pressure setting, which would dynamically see opacity based upon how hard you press with the stylus. But that's merely not possible without a tablet.