What i used to find hard to judge was the colour of my mixed wet paint on the palette, versus the dry finished product on a miniature.
What i started doing was custom mixes in empty paint pots. They keep the paints good to use for years, and they are always the same colour every time you go back to them, and they save time in the long run too. Its really handy for painting and entire army, but they are also handy for doing one offs once you build up a few of them. If you look at most of the citadel 'eavy metal guides, you will see so many custom colours used.
I have a friend who works at a hospital so i use syringes to mix my paint in exact quantities. This way i can get my mixes perfect 100% of the time.
Here are some example of 'how i roll'