Yes, Snowcaps and Fewspots are said to be homozygous, and therefore 100% appaloosa producers. But remember, there are many patterns of appaloosa -- not just the "spotted" ones that you might normally call appaloosa.
The best place to learn about appaloosas, is to join 'The Appaloosa Project' -- where molecular level research is currently being done, identifying the LP gene, modifiers, PATN-1 (the leopard family white pattern gene) etc. A quote from The Appaloosa Project main page states: " To understand the importance of the search for this gene, you must first know what LP is and its significance for breeders. LP stands for “leopard complex”, and is the name assigned to the gene that a horse must have in order to look like an Appaloosa. "Leopard complex" refers to a large group of patterns, any of which may occur when LP is present, and “leopard” simply refers to the unique form of spotting common to them. LP acts as an "on-off" switch that when present in dominant form, activates Appaloosa characteristics (striped hooves, mottled skin, white sclera) and Appaloosa roaning (Sponenberg, Carr, Simak, Schwink. 1990. Journal of Heredity, 81: 323-331). It is hypothesized that LP acts together with modifying genes to produce variability in patterning."
So, although a horse is homozygous -- snowcaps and fewspots -- their offspring can show a variety of different color patterns. Some roan appaloosas have also been found to be homozygous.
For example, there can be a very small blanket, 10% up to 60% blanket or lacey blanket. (The following percentages and 'pattern names' come from the charts produced by The Appaloosa Project).
When the white, spotted blanket goes much over 70%, these horses are called near-leopards up to full leopards. Of course these two patterns (blankets & leopards) are the most recognizable 'spotted' appaloosa patterns.
Then you have snowcaps -- whose 'unspotted white blanket' could be as little as a 10% pattern, up to 60% white blanket area.
When the 'snowcap' extends to 60-80%, these horses are identified as in the near fewspot range, and at 80-100% they are called fewspots. These appaloosas may not visually show much in the way of 'spotting' however a close clipping and a wet-down will usually show there is a leopard pattern to the skin beneath that does not visually grow the usual 'spots' in the coat.
Since I can only show my own horses, here are 3 horses that are homozygous for appaloosa color/patterns:
This colt had a small snowcap blanket
This mare is a snowcap with roaning
This filly is a near-fewspot/fewspot, and I'm including the 'spots' that could only be seen at the skin level
And just so you understand, even a fewspot can produce what appears to be a visually 'solid' foal at birth, but who 'grows' into her appaloosa pattern -- making her sire a 100% color/pattern producer. This filly, out of a Palomino Fewspot sire and an appaloosa roaned dam was born solid black (with appaloosa patterned hooves), but matured into a much more 'visually' recognized appaloosa pattern (picture taken by her new owner as a 2 year old).
