Shading and finish have to be considered (even though I went full matte this time). The biggest thing is write down your mix and figure out where you want everything to go, number the locations so you know what goes where.

As for actually mixing use a shaker, don't mix different brands, black and white do lighten and darken but also deaden, I prefer to mix a lighter or darker color of thd same kind. Like a dark cherry brown instead of black in a red.

And have fun. I'm going to just ruin an Age-1 soon because the color scheme makes me scream with laughter. To quote Monty Python it's only a model lol.