Start removing parts that would have the original color under them. Maybe in other areas carefully sand through the layers of paint until you get to the original layer if the car wasn't stripped to repaint. All of the fenders would have been black. The '28-'29's had a few colors, they added a few more color choices in '30 and for the '31 they had 38 colors available. What year is your pickup? The Model A clubs (mafca.com and modelaford.org) have an excellent chip book available that you might be able to borrow from a restorer. This might be a good question for ford barn.com. Charlie Stephens
If I remember correctly, Commercials could be ordered in any of the available colors for 1931. Here's a link to a site showing the available colors: http://paintref.com/cgi-bin/colorco...1&manuf=Ford&model=All&action=Get+Paint+Codes Your fenders and aprons will be all black, anything above that can be whatever. Where you see "Lower Color" that is between the beltline and the aprons.
As stated before, look under the areas that probably weren't removed during a repaint, or never got faded. I found the original color for my roadster under the dove tails in the door jamb
It is even better than that for 1931. You could get any of the colors available for p***enger cars plus some that were unique to the commercial vehicles. Charlie Stephens