We have owned many grey horses – mostly Arabians and part-Arabians – over the years and although I love the look of a dark, dappled grey none of ours attained that. Our Welsh/Shetland pony cross was beautifully dappled for a couple of years, but we purchased him at the height of his dappling and he soon greyed out entirely.
Gray horses are born any number of colors – bay, black, chestnut – and 'grey out' over time. Dapples on a grey are part of the greying stage where there is a pattern of light spots surrounded by darker rings. While some horses may hold this color for years, others pass through it very quickly...like most of ours did.
With many greys there tends to be a period of semi-dappled, semi-grey where the horse appears dappled – mainly on the points and rump – but it's a very faint dapple pattern. The next common 'phase' of greying is the absence of the dapple pattern. The horse can either appear pure white or a cloudy grey.
The last phase of greying out is the flea-bitten grey. These small brown spots tend to creep into the horse's coat at varying degrees. Some grey horses may only ever have a slight smattering of 'flea bites,' while with other horses they can become extremely thick, appearing over the entire body of the horse.
It's also not uncommon to see a non-grey horse with a dapple pattern on its coat. This is an entirely different form of dappling and is caused by feeding and the health condition of the horse; not genetics.