I usually don't bother with this when I'm painting single models - BUT, armies look really nice when they're painted with a uniform colour scheme. Some variances are okay, but generally, you'll want them to be similar. Those similarities are what you should keep track of.
For example, I have notes for every version of the Eldar Alaitoc armour I've painted, and their accessories. (For reference, there've been 3 - heavy armour w/ chainmail, a newer style similar to aspect armour, and the newest sleeker style.)
I spoke about this sort of thing a little while back when I discussed Organizing Colours While Painting.That article was about planning ahead and pulling out all the colours for a mini before you get started AND lining them up in the order you'll use them. This will give you an idea of what your mini will look like, what colours you need, how the colours will look together, etc. Essentially, it's this order that you want to keep track of.
So, what sort of notes do I mean?
As you can see, it's just a point form list.
After some of the lines, I make other notes in brackets - usually what's a wash or h/l (highlight) but sometimes one or two more words about how it's applied, mixed, blended, etc.
When I mix paints, I just use a plus sign " + ". If it's a 50/50 ratio or not too critical, I tend to not make any special notes, but I'll use " 1 : 2 " or whatever if the mix is a little more specific. Sometimes when I'm mixing I'll even slap a bit of colour on the paper so I've got something to aim for.
Another reason to do this, is it allows you to paint models one at a time, rather than assembly line stile. If you paint one colour on every model in the unit, then move onto the next colour, and so on, YOU WILL BURN OUT. It also doesn't allow you to get creative and blend. Keeping notes lets you focus all your attention on one model at a time, and you'll be healthier for it.
I'd never really painted Orcs before, but I had a good time doing it. There were only three of them, but I knew that if I came back to paint more down the road, I'd want them to look similar. (Though I'm starting from scratch with my Hordebloods as I want the army to have it's own style)
My Eldar style was much bolder, less blending, but here's the notes I kept for them:
The following are my notes from that project. You'll see I crossed things out as things change, and put some colour down as targets for later.
Excuse the water damage... there was this flood, and I'm lucky to even have this notebook still.