Thank you for the comment and the questions @minnowforlife!

I use thinliners like those for clean outlines

And ballpoint pen like this for lines that need more variations, ink is more oily.

Both have waterproof ink but to be honnest I don't know what type of ink it is exactly.

You can find a bunch of watercolour tutorials on internet. For gradients for instance, you might do it with a first layer that you let dry then adding a succession of others. But that'll be more of a "fake" gradient, that'll still look nice, all depends the effect you look for. But the "real" technique for that is to lay the colour on one side, then another colour or clean water on the other side (without having them touching each other) and then blend them. You have to be pretty fast for that technique, you might even need one brush (one with each color) in each hand to do it before it's dry.