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Happens a lot with highland ponies Because Dun and grey (with primitive markings) are the most common colours... As far as I undrestand it, grey is a dominant alelle, so if you get one grey gene, your horse is grey, it overwrites other recessive colour genes. I'm probably the worst person to explain this

It doesn't just overwrite the recessive genes, but also the other dominant genes.
That's because grey actually isn't a color gene, but a genetic 'mistake' that causes the horse to turn grey at a very early age instead of turning grey from old age.
With normal dominant color genes, ALL the dominant genes a horse carries show. For example if you have an amber champagne dun tobiano, wich is black + agouti + champagne + dun + tobiano, all the genes will show an infuence on the color of the horse. Grey overwrites every color and the other colours can't be seen anymore.