I'll chuck out something that has saved me some time (and I hope to turn it into painted armies over these next few days).
I spent a few odd nights cleaning and assembling and basing a mass of models. When it came time to paint, I basecoated the entire force in black, and then broke it up into:
Units that are mostly brown
Units that are mostly blue
Units that are heavy machinery (gunmetal)
Special characters that are none of these things. (grey)
I grabbed the mostly brown units and did a brown overspray on at batch of figures.
I followed it up with a top down spray of a that brown plus some bonewhite.
I was able to use them in a game straight away as I could now identify them based on pose and some detail.
I plan to go back for the blues, the gunmetals, and the grey/white, then move through them picking out the faces, weapons, army badges/shoulder pads etc.
It makes for quick work, and I leave myself in a good position to pick up again if something interrupts my painting flow.