Colour is genetically determined by DNA, so it is determined at the point of combination of DNA of sire and dam. In a way, one could say that is it determined before conception, with some variance at the time of conception, as certain colour parents will only be able to produce certain colour offspring, but the combinations will be variable in most cases, so it's not until the specific parental DNA combines that the final outcome is determined.
A-ha! Thanks Jos, that's pretty much what I imagined Now we know why for eg. Appaloosa colouring is so.... unfathomable
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