I must have misread(?) that your car had standard interior - so my initial search was for standard interior cars;
Before hitting "send" to post my reply, I reviewed your PHS created window sticker, and can see that it's a code 431 (Deluxe/saddle) interior car, so I am including sharing
BOTH standard and deluxe interior images below:
Here are two examples of 1973 Firebird with
standard saddle interior (code 331):
1973 Formula - Barn find (unrestored)
1973 Formula modified/restored
Here are two examples of 1973 Firebird with
deluxe saddle interior (code 431):
1973 Formula - barn find (unrestored)
1973 Formula - mint/original
(
larger interior image)
In addition to lighting which might be attributed to colour variation, you will see legitimate colour variation on original interiors - often the glove box door differs from the dash pad;
And you will also typically see variation between the rubber (door panel) arm rests, and the molded plastic armrests they attach to - as well as the vinyl covered door panel top halves.
If you ever see a car where the interior is all one shade of brown, rest assured, that someone spent time doing that.