Not a notebook, but I do two things:
1. I keep an Excel file for each project, with numbered steps, colours desired, paints used, and any comments (needed thinning, etc.). I print this and update it in pencil as I go, and then revise and save the edited version for each project. This is very useful, especially for units like Napoleonics where only some things (e.g., facings) are different, and also because similar units will have a ready record of all paints used over time.
2. I make my own paint chips from squares of matt board. I store these in collectible card sleeves, and write on the back what projects I used them for. This gives me, at a glance, a page of blues or reds to look at in a reasonable size, and a pretty good sense of how they cover and how they look when brushed on. It also allows me to pull samples and compare the look of colours, much like a designer would, to see if they 'fit'. Most recently, this was for reds and blues on a Hesse-Darmstadt unit for 1809.
Not an elaborate system, and I rarely mix colours, being a rubbish painter, but it works for me.