homozygous grey is more likely to develop melanomas than a heterozygous grey. I believe that heterozygous greys are also more likely to develop the "flea-bitten" look. Horses that are aa at agouti (as opposed to Aa or AA) are also more likely to develop melanomas. This was all discovered when they developed and came out with the test for grey. Notice, it says most/more likely, not absolute.
So...*is confused*...since my mare has tons of melanomas AND is heavily "flea-bitten", she's more likely to be heterozygous for grey and aa for agouti vs something else? Neat. :)