When I was an oil painter I started quite a few paintings with a monochromatic underpainting first, and those are among my most successful oil paintings.
Recently I saw another artist using diluted ink to do the same thing as an underpainting which he then glazed with watercolour, and I just had to try it.
This scene is from my local beach – the large pipes are (I think) stormwater outlet pipes, and the path leads to the ocean-side swimming pool. I drew the painting in pen first and then used several layers of dilute ink to build up the values, until I had this;
Then I glazed it with watercolour and added more details and hatching with pens, until I had this;
I’m not entirely convinced the end result was worth the extra steps – I could have achieved the same result with pure watercolour, I think – but that said, it did give me plenty of time to really consider where I wanted the light to fall, which I sometimes struggle to do when also trying to consider colour.
I’ll do a few more in this style and see if that nagging doubt remains!