Sometimes, after you've painted a miniature you need to go back and fix a mistake you hadn't caught the first time around. When you revise your paint job, though, you're usually filling in something that's missing or concealing something that isn't meant to be there. That means you have to match the exact colour of the paint you applied months ago. Do you even remember what paint you used last week, much less months ago? I don't. Luckily, there's an easy solution.
I had a really bad habit, early on, of using any random paint I could find. When I first started painting, I was using one of the Paint and Tool Starter Kits from Citadel. It's a great kit, but it's pretty specific to a certain range of Warhammer miniatures. For instance, mine had no green paint in it. While painting some monsters, I wanted some green paint. The Warhammer store is an hour away, so I just raided my partner's teaching supplies and grabbed a $3 acrylic paint tube meant for children's crafts, watered it down a little, and used it.
It actually worked really well, so I started grabbing cheap acrylics from my partner's stash whenever I needed a colour I didn't happen to have. As a result, I can't begin to guess how to match the paint of any of my earliest 6 miniatures.
Later, when I discovered that you could mix paints, I started mixing some of my Citadel paints to invent new shades of colour. I'm sure that's fine to do, but I didn't write anything down about how I achieved any given shade I created. As a result, I have no idea how to match the colours for the second set of miniatures I painted.
Since those days of exploration, I've learnt to help my future self. Now, when I need a specific colour, I buy that colour. I know there's power in being able to mix paints well, but for me there's safety and convenience in letting the experts define a colour for me.
Buying Guilliman Flesh solved all my skintone problems. Sticking to Macragge Blue for deep blues and Pylar Glacier for light blue means I only have two paints to choose from when I need to touch up a blue region on a miniature. Having defined choices makes it possible for me to understand what's on a miniature.
When I was just painting miniatures from board games (I had something like 52 miniatures between Pandemic: Reign of Cthulhu, Wrath of Ashardalon, and Warriors of Krynn), it never occured to me to jot down notes about the colours I was using on each model. Once I got some miniatures from Games Workshop, however, it seemed pretty obvious that I ought to record the paints I use. After all, the booklet showing you how to put the models together provides a manifest of which paints you can use to mimic the models as shown on the box. Following that example, it was pretty easy to just jot down what colours I ended up using right in the booklet. When I was done, I put the booklet back in the box. It was no longer documentation for how to assemble the miniatures, but documentation for how each one was painted.
I imagine that people who know how to mix paint could probably record a recipe for their custom mixes. I don't know how to do that, but if I started mixing paints I'd probably figure out a way to measure the different parts of each colour, and record it for the future.
I think the key is documentation. On one hand, that seems like extra work, but in practise painting is pretty methodical. You're usually waiting for a coat to dry, so take that moment to jot down what paint it is, exactly, you're waiting on. I jot these down alongside the assembly instructions, so you don't even need a special notebook. Just keep a pen handy, and write down the colour name.
When I first started painting miniatures late last year, I thought the workflow was: Acquire a miniature, prime it, paint it, use it in a game. I wasn't wrong, but there's a potential additional step that I hadn't anticipated. It was hard to identify at first, because it never happened in the beginning, and even when it did start to happen it only happened some of the time. The missing step is revision. Make revision easy for yourself by using paints only as they're provided by the vendor, and by writing down which paint you used for each model. You'll thank your past self, or at least I do. There are probably other ways to manage this issue, and I'm not saying mine is the most elegant or liberating, but it is convenient, simple, and effective.
T'au soldiers photo by Seth Kenlon. Creative Commons cc0.