I got into the habit of drawing with pen rather than pencil as a teenager, due to my math class of all things. With math courses, its important to "show your work," and part of that includes showing the mistakes. Partially, so that the teacher understands where your logic is flawed; mostly, to get credit on the parts you did get right.
My teachers mandated that work be done in pen, so you wouldn’t erase the partial credit you could get. And, I stopped carrying a pencil with me.
A few years later, I was taking a printmaking class. While I did know that most printed material is actually an optical mixture of cyan, magenta, yellow, and black (CMYK) inks, it was the first time I ever did the manual work of creating colors from a limited set. (I was, and still very much am, a person that adores collecting colors.)
This led me to updating my pen sketches: with the addition of rose, blue, and yellow micron pens to my sketching kit. And, I tend to use these in a mix of two ways: to 'see my work' and to create more colorful sketches without needing to bring so much with me.
To see my work, I overlay gradually improving sketches in increasingly darker hues: starting with a super rough one in yellow; refined by an overlayed pink sketch; then a more refined blue sketch; and a final 'complete' sketch in black.
As you can see in the rough color wheel, I can also visually mix a range of different colors using hatching. So, now, I could try to get a rough approximation of hues using optical mixing, while relying on line density to approximate the value.