Sorry, I've forgotten how to identify it. Flea-bitten grey is a version of grey where some pigment cells become reactivated and little dots or speckles or flecks of the original coat color start growing back into the white coat. The fleabitten color can become so total that the horse will begin to resemble the chestnut bay or black that they once were. Again the mechanism is unknown it is probably a modifier gene such as the one or ones which control how fast the greying process takes place. I have an entirely unstudied theory that flea-bitten color can at least be limited in extent by keeping the horse out of the sun.
Bloody marks are areas of pigmenting that never grey out on a grey horse.
They can be quite extensive or they can be very small. The genetic mechanism is again unknown but is unique to Arabians and said to show antique or ancient blood. The occasionally white body spotting that shows up in Arabs and leg runs, white leg markings that continue in a streak up the leg sometimes past the hock, are also said to show old blood and may be related to the Sb sabino gene or it may be something unique. If related to the sabino gene it may explain the very rare parti-colored horses that are occassionally born to purebred Arabian parents. As with any arbitrary system of classification it does little justice to the beauty it attempts to describe.