Thank you for your example
You have two possibilities
Firstly, launch layers panel [L]
BLENDING MODE :
Multiply modeif you want to keep your strokes over and colorize easily under them it is possible to use the multiply mode.
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1 - if you are in color mode normal you will have something opaque (Note: the darkness of your stroke will define the opacity)
2 - but if you choose the multiply mode you'll be able to draw under your dark strokes easily
BLENDING MODE: Normal to
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1 - You draw with grey value
2 - and create a specific layer for color
3 - Switch the blending "normal" into "color" mode
4 - And set the opacity
With this technique you'll have too use saturated colors