From information I've gathered, Pied and White are two mutated versions of the same "normal" gene. Since peas can have only two copies total (some combination of "normal", Pied or White), then the ONLY possible combinations are (with names given to resulting phenotypes in parentheses):
1) two copies "normal" ("Normal")
2) two copies Pied ("Dark Pied")
3) two copies White ("White")
4) one copy "normal", one copy Pied ("split to Pied")
5) one copy "normal", one copy White ("split to White")
6) one copy Pied, one copy White ("Pied")
Having one copy of either Pied or White, and one copy of "normal", results in a bird having a few white feathers. Though I'm not 100% certain, from what others have told me, being split to Pied results in "less white" than being split to White, and in the split to Pied birds, the white is restricted to the flight feathers and (sometimes) on the throat. Being split to White is more random with regards to where the white feathers are -- sometimes in the same spot as in split to Pied, but could also be a random white feather or more elsewhere. Having two copies of White results in an all-white bird. Having two copies of Pied results in Dark Pied, with white feathers being basically restricted to the throat and wings -- but more white in those areas than in "split to Pied". When a bird has one copy of White and one copy of Pied, the result is a variable expression -- more white than Dark Pied, but less white than White. Birds with "lots of white" are called "loud Pieds" while those that are less white (but still more white than Dark Pied) are called just "Pied". I think the variability might be due to:
1) simple random variable expression of the White gene
2) variability of expression of the White gene due to maternal versus paternal inheritance -- perhaps one chromosome of the pair is "more active" than the other depending on from which parent it was inherited. Such a phenomenon can be found with other genes in other species.
3) other unknown and subtle modifier genes which affect "how active" the particular White gene is as an "eraser" of pigment.
4) some combination of any or all of the above
White Eye is a separate gene, which is visible in birds having only one copy (some ocelli are white, some are colored) or two copies (all ocelli are white). If a bird's whole train is white, anyway, one can't tell if it has one, two, or no copies of White Eye. Thus the White birds from Silver Pied X Silver Pied would carry the White Eye mutation, but visibly are the same as a White bird NOT carrying that mutation.
When a bird has one copy of Pied, one copy of White, and two copies of White Eye, the result is a bird with even more white than a Pied bird. But because there seems to be a range of "how white" a Pied bird can be, there would thus also be a range of "how white" a Pied bird with two copies of White Eye will be. It appears that there may also be another gene which results in the "powdered" saddle that many consider an additional and necessary ingredient for what should be properly called "Silver Pied". I don't know if this is an allele (or "other version") of White Eye, or a separate gene altogether. But as I keep looking through breeder websites, I am seeing some birds which display the "powdered saddle" even though they are not Pied, much less "Silver Pied". So perhaps the "Silver Pied" phenotype requires 4 mutations to be present -- White and Pied (which are alleles, or two versions of the same gene), and White Eye and the "powdered saddle" mutation (which MAY be alleles, or may be separate genes). It's hard for me to say for sure without conducting test breeding to answer "if this is the case, then that should be what I see if I cross this and that".
Birds are pictured as "White Eye" on various breeder websites which have either one or two copies of the White Eye gene (I use the term "Single Factor" versus "Double Factor"). This is apparent by looking at the trains -- one copy results in some white ocelli, and some colored ocelli, while two copies results in all ocelli being white. So birds called "Pied White Eye" may have one copy of Pied, one copy of White, and only one copy of White Eye (thus some ocelli are NOT white). Or perhaps they have two copies of White Eye but lack the "powdered saddle" trait (whatever causes it). Or perhaps they HAVE the "powdered saddle" trait, but also display some colored ocelli (thus are Single Factor White Eye).
The point is that they seem to be "one ingredient short" of everything required to be "Silver Pied". But at the same time, without a consensus on how a Silver Pied must look, and what genes it must have, there will be gray area from breeder to breeder. You might understand "Silver Pied" to be one thing, but the breeder from whom you buy chicks thinks it's something else, and when your newly-purchased birds finally feather out, you may experience disappointment. The breeder "believes" the birds sold were Silver Pied. You "believe" they are not. If there's no standard, how do you decide who is correct?
