During our stay in Venice, we took a watercolour painting class! It was really interesting; very different from the acrylics I am using!
We start by making a pencil drawing of the composition. We also add values (shading) that will be converted in colours.
The main difference for me was that the work with colours is in transparency. We do not 'block' the paper with the pigments, but rather do different layers of transparent colours. It's more like combining filters and light than opaque colours on a support (the light being the white of the paper).
There is no black; the shading 'grey' is made by mixing the colour with its opposite colour in the colour wheel (for example shady red is red+green, making a brown-ish tainted with the original red colour).
The key things I learned were:
1/ You need to use lots of water
2/ To make interesting marks or stains, you need to use lots of water
3/ Need erasing? No problem, just rub with water
4/ In doubt, put more water
5/ Water is the key