Yes - this was mixing two Rusto colors.
Now, as for adding pigments.....
I don't know how/if they'd affect the paint at a chemical level. In theory, it should be OK - as long as you had the right pigments.
But why go to that trouble/risk anyway? Rusto comes in all the primary colors, plus black and white, so you should be able to mix up any color you like.
To try out the colors before you take the plunge and buy some big Rusto cans, maybe you could experiment with some of those little modelers' enamels?
What color are you trying to achieve?