She appears to be going thru the many coat color changes that all foals go thru. It can be dramatic - even with known colors.
The mare that is "black-ish" is hard to tell what color she truly is, but she could be black or she could be a really dark silver...
I still hold that this filly is either silver black (silver dapple, dapple, silver, chocolate, choco-black, taffy - depending on which breed, which country, which area of the country you are in) or silver bay. The "silver" gene only dilutes the color black. It is not yet known how the "shading" works. Black horses fade or "bleach" or "color out" - with sweat and sunlight. So do silver horses. Black horses have different shades as do bays. Hence, different coloring before the silver gene is applied and before it fades. I've found that nutrition and weather play a part in the coloring, too. Colors can be enhanced by fat and feed additives. What's good for the coat will change the color/shade.
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You can breed two bay horses together and still get a chestnut or a black - depending on the genealogy of the two bay horses. You can also get no "red" foals from two bay horses. Depends on whether they are homozygous for black or not - and homozygous for bay or not - not what the two parent horses look like.
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Here is the latest picture of our 2013 filly, Bunny. She is homozygous black, no bay agouti. We haven't tested her yet - don't know if she is homozygous silver or not and don't know if she is homozygous tobiano or not.
Here is a picture of 3 silvers - in different shades. The larger mare in the back, mostly hidden, (Flower) is a silver bay tobiano (E?, Aa, Zn, Tt). She was not tested - so don't know if she's homozygous (EE) or heterozygous black (Ee). She can only have one silver gene (from sire), 1 bay gene (from dam) and one tobiano gene (from sire). The mare on the left , (GG) is a heterozygous black, no bay agouti, silver tobiano - tested (Ee,aa, Zn, Tt) and one dam tested as well (she's homozygous silver). The filly on the right is Bunny, not tested but both parents are - a homozygous black, non bay silver tobiano (EE, aa, Z?, T?). Flower and Bunny are 1/2 sisters. GG is totally unrelated to them.
And I know I posted a pic of our lots of white tobiano, silver bay colt (Dandy) with all his winter hair taken in December. He's hard to tell he's a silver bay - as he has no "noticeable" black points (he's pinto white where he'd otherwise be black) EXCEPT for one very small area in his forelock/bridle path area. I know he's silver bay because both of his parents are tested and he can only be homozygous black (EE) - not heterozygous black (Ee) or chestnut/sorrel (ee). His dam only carries one bay agouti - and if she hadn't given it to him he'd be black not "red". He only appears to be red due to the work of the bay agouti gene (Aa). If he were kept a stallion and bred to any color mares - no red foals (he can sire homozygous black, heterozygous black, bay and silver). This pic is almost current and I'll try to get one of him tomorrow that shows him almost completely shed out.
That bit of forelock is flaxen or black diluted by the silver gene and is the only spot you "see" his silver gene.
And in this picture - Dandy is standing next to Echo. Echo is not color tested either (yet). BUT his parent are. He is heterozygous black (sire is black EE, dam is chestnut ee), no bay, obviously tobiano but could be TT or Tt - depending on which pattern his dam gave him - T for tobiano or t for solid. Echo's sire gave him one tobiano gene since his sire is TT.
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What did you end up naming your filly? How is she doing? She's pretty darn cute!!