Grey x grey is more likely to produce a grey, but is not guaranteed to, depending on the genetic makeup of the parents.
The two really basic rules of colour genetics, which I believe the Jockey Club uses to do a quick double check of parentage are:
Two chestnut parents will always have a chestnut foal.
A grey horse must have a grey parent. (And this has caused some confusion in the past when grey horses are registered as their birth colour, never get their registration updated, and then have grey foals.)
The chestnut rule is simply because chestnut is the most recessive of horse colours and two chestnut parents are not bringing anything else to the table. (Because equine colour works across multiple genes, they may actually be carrying genes for bay or black, but those can't expressed unless they are bred to a bay or black.)
Grey, on the other hand, is the most dominant of horse colours and it overrides everything (IIRC, Tesio considered it a pigment disorder instead of a colour). It can't 'hide' and pop out later in a line, and a homozygous grey (a horse carrying two copies of the grey gene) will only have grey foals, as every foal it has will be receiving a copy of the grey gene. However, a heterozygous grey (a horse with one grey gene, and one non-grey gene) will pass the non-grey gene down to approximately half its foals.
Tapit is clearly heterozygous, and, from her pedigree, Silver Screamer is as well. (Her dam is chestnut, and you can't inherit a grey gene from a horse that isn't grey.) So when you breed two heterozygous greys together (Gg x Gg), 25% of the foals will be homozygous grey (GG), 50% will be heterozygous grey (Gg), and 25% will not be grey (gg). So this filly is in that 25%; she's inherited both her parents' non-grey genes.
The other possibility is that she is grey, but is greying out slowly and her registration hasn't been updated. However, Tapits tend to go grey unusually fast (I don't know why), and chestnuts tend to grey out faster than black-based horses, so I think it's unlikely in this case. Would have to see her, though.
That was probably more than you wanted to know, but I hope it was useful.