From Mini Painting Wiki
Formula P3 - that's Privateer Press Painting, usually just called "P3" - is a range of hobby supplies commissioned and sold by Privateer Press.
The paints range is sold in ten sets, each designed for a particular faction or troop type in the Warmachine or Hordes ranges, and as individual paints. The unique selling point of the Formlua P3 range is that it's made with a liquid pigment rather than a powdered solid pigment, although how unique this actually is proves hard to determine as other ranges have been close-lipped about the exact formulations of their ranges. Although the manufacturers of P3 claim that most paints will cover even a black undercoat in a single application, they still include a number of "base" paints with a higher pigment content that are explicitly designed to do so.