We only have three different alleles of a single gene to determine color; sable, black with tan points, black without tan points. Labs/Poodles have multiple genes that determine color. The chart is pretty big for them.
If you think of Sheltie colors as three different pots, it's easier.
Color (brown or black)
Modification (merle or non-merle)
White (piebald, white factoring, irish pattern)
It all inherits separately, so you can pretty much have any combination.
Looks simple, and it kind of is.
You have 3 "base colors". Bi black, tri color & sable.
You have modifications: the merling gene & white factoring. & the Irish pattern (which most Shelties have.)
Pure for sable = ay/ay = can only produce sables
Tri-factored Sable = mahogony sable = shaded sable = ay/at = can produce sables and tri's
Bi-factored sable = ay/a = can produce sables or bi's
Tri-color = at/at = can produce tri's
Bi factored tri color = at/a = can produce bi's & tri's
Blue merle = at/at M/m = can produce tri's & merles
Bi Blue merle = a/a M/m = can produce bi's & merles
Bi-factored blue merle = at/a M/m = can produce bi's, merles & tri's
Bi black = a/a = can produce bi's
Pure for Sable merle = ay/ay M/m = can produce only sables or merles
Tri-factored sable merle = ay/at M/m can produce sables, tri's or merles
Bi-factored sable merle = ay/a M/m = can produce sables, merles or bi's
Double blue merle = at/at M/M = can produce only merles
Double bi blue merle = a/a M/M = can produce only bi's & merles
Double pure for Sable merle = ay/ay M/M = can produce sables & merles
Double tri-factored Sable merle = ay/at M/M = can produce sables, tri's, merles
Double bi-factored Sable Merle = ay/a M/M = can produce sables, bi's Merles
ay/a (bi-factored sable and white) and "breed" her to any dog on there. Let's go with a tri-factored sable and white (ay/at). So, you could get pure-for-sables (ay/ay), you could get bi-factored sable and white's (ay/a), you could get bi-factored tri color (at/a).
It only takes 1 sable gene for the dog to be sable, since it's a dominant gene. It takes 1 gene for a dog to produce a tri-color or a bi-color (called bi or tri factored), but 2 genes for a dog to BE a tri-color or bi-color (like a/a or at/at). Merling (and white factoring) is like sable and white, 1 gene for the dog to be a merle, but 2 genes for the dog to be a double, or homozygous merle.
I've noticed, the more I go on, that Sheltie colors are like horse colors (which is to say that the genes are done in the same manner, like they usually are!) Except, IMO, horse colors are harder because of the wider range of colors (much like looking at a whole, much like looking at dog colors as a whole instead of looking at just Shelties.)
Horses have 2 base colors: Black *dominant* (E) & red (e).
Then they have modifications: Bay (several kinds, A, At, A+), Gray (G) & Dun (D).
Then they have dilutes: Cream *palomino, buckskin, smokey black* (Cr), Pearl (Prl), Silver *related to the merling in dogs* (Z) & Champagne (Cmpg *this is not the right letters for that*)
You can only have 1 base color. You can have all modifications. You can have most dilutes. By that, I mean the genes. You can have a black horse Ee with Bay *agouti* AA, with dun Dd, with cream CrCr. So far, we have a perlino. Then, you add silver Zz. So now we have a Silver perlino.
Pearl is on the same allele as cream, so you can either have 1 cream 1 pearl, 2 creams no pearl or 2 pearls (or none of either of them). Except for pearl & cream, you can have dilute & modification on dilute & modification. It wouldn't be easy, but it would be possible.