To be honest, the only way to get consistent paint colour is to use something you can buy off the shelf — and even then, it could be the manufacturer changes the formula and you end up with two distinct shades …

[ATTACH=full]132911[/ATTACH]

This is Takom's M29 Weasel. I started spraying it with the bottle of Mr. Aqueous H78 Olive Drab (2) on the right. That ran out when I was about three-quarters finished, so I opened a new bottle of the same (on the left) to spray the rest, and, well, you can see the result  In the end, I had to respray the whole model with the new paint.