i'm sure it was exactly like a mustang. what you'll see is primer color all under the car(red-oxide, black or grey) and then you will see body color wafting up underneath the car from the outer edges. the color will be somewhat heavy and get lighter towards the middle of the car.
MCA Gold Card Judge(64.5-65, 66)
'00 GT Conv, triple black
As Charles described The primer sealer could be one of many colors in 69 since they were mixing in great amounts of left over exterior color. Colors will vary depending on the week and the factory. Dearborn seemed to produce the widest variety with reds, dark purples, and so one so look real close at the drive line tunnel or the rear end area. Without going into great detail over the top of the primer sealer (DO NOT USE Primer use primer sealer) you will have exterior overspray (greater at the outer edge of the body (sides and rear) and over this black over spray (less that the body color) along the pinch welds (this car take place while you are "blacking out" your rocker panels) You will also find that this process normally blacks out things like the rear of the front frame rails, the forward rear spring mount area. Then when the firewall and inner fender panels were painted black (where and how much depends on the factory you car was built at) you will see black overspray at the forward areas of the floor pan, trans tunnel and foot boxes.
Though some were produced with the pale milky red oxide, but 69 the color varied greatly depending on factory and the week it was assemblied from the original cars we've documented.
For example I have a Dearborn 69 car that is so dark it is almost black. With a strong light you can see that it is a dark purple. The car does has some additional red oxide but that is because of the factory modifications they did and the early Boss 302's.