"they are different numbers because they are different colours".Thats basically what i have said, so the theory that all panels are reds or begin as red is incorrect.
I think Ducati did it occassionally at the end of production runs though, if the story I heard from a Ducati dealer is true (taken with a pinch of salt).
Ducati do supply a plain panel though, but again different part number. The part number will tell all
Long live Yellow Ducatis, the best colour by far.