I no longer work with wood for model projects, but do play with formulae for paint colours.
My process is to start with a small amount of the colour that's close to what I want, then add "brushloads" of a colour that I think might adjust it to the colur that's desired. I keep an ongoing record of the colours and amounts used, until I get it right. I then copy that formula into a book for future use.
You should be able to do a similar process with a small measured amount of alcohol, then add a measured amount India ink or shoe polish. Continue to adjust (either up or down) the amounts of each component you're using, until the finished sample looks correct to your eye. Keep a record of those amounts as you work.
All that's then required is to translate those small amounts into larger useable ratios (make sure to keep a record so that you can refer to it in the future, too).
Back in the early '80s, I mixed a formula for painting models of some prototype diesels, and when I finally got it right, painted a dab of it on a coloured photo of one of the real locomotives. Unless the photo was turned at an angle oblique to the light, the dab of paint (a dead-on match) was impossible to see as such, only because it was a matte paint on a slightly gloss surface.
At that time, properly-coloured models of those locomotives were not commercially-available, but a couple years later, they were. (I was glad of that, as I'd done over 70 of them for fans of that particular railroad, and was getting tired of the work.)
Forty-odd years later, I got several requests for that paint-mixing recipe, luckily saved and readily at-hand.
Wayne
