They both had red and grey, but the placement of the 2 colors was slightly different.
The 501, 601 and 701 series were known as the Workmaster Series, and they series had an all red hood with gray on the grill, wheels and fenders.
The 801 and 901 eries were known as the Powermaster Series and had red on just the center of the hood, not on the entire hood, and on the grill trim. The rest of the hood, and the fenders and wheels were grey. See John Smith's "Identifyng old Ford Tractors" web page for more details by CLICKING HERE
Edited 11/8/2012 6:16EST - fixed error pointed out below. - Sean This post was edited by Sean in PA at 03:16:33 11/08/12.