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.
![Image](https://trello-attachments.s3.amazonaws.com/50bf3f4076a627ab3200a072/5178e79e7330623477008b6f/a5363839a9a2e1a531a4fa50c1dac481/layerblending02.jpg)
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
Color mode![Image](https://trello-attachments.s3.amazonaws.com/50bf3f4076a627ab3200a072/5178e79e7330623477008b6f/7ad42c89a328ce226091055287c35a06/layerblending01.jpg)
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