It's all in the genes...
Paint Horse breeding has its own complexities due to the genetics behind the colour patterns. These marking are more than just "white spots"!
Today's terms referencing these patterns reflect not only these markings, but the genes deemed responsible for them. APHA formally recognizes three main Paint Horse patterns: Tobiano, Overo, and Tovero. Solid horses, or horses with "incomplete" (insufficiently-sized) markings registered under APHA are referred to as Breeding Stock horses.
Further qualifications can be made to these three main types of patterns. For example, a minimal Tobiano or Overo is one that shows very little white. By contrast, a loud, extensive or maximally expressed pattern is one that shows a lot of white. In describing the colour balance of the pattern, breeders might refer to a Paint Horse that is "30% white", "70% white", or "50:50". "Medicine hat" is oft used to describe a Tobiano, Tovero or Overo with an all-white head, except for a splash of colour on its ears and/or poll.
Terms such as piebald (referring to any black horse marked with excess white) and skewbald (referring to any non-black horse marked with excess white) made with reference to Paint (and pinto) horses are generally considered obsolete, now that coloured horse genetics are better understood.
Paint Horse genetics are complicated because some facets behind the patterns -- notably Overo -- are not yet fully understood. I've attempted to summarise my understanding (and opinion) of these in the pages which follow.