Mike Brock, misunderstanding the meaning of "color chip", wrote:

Flat black? Have you studied the...uh....floor colors of these
cars? Wouldn't you be better off coming up with a...uh...chip color
[ somewhat akin to "buffalo chips" that were used supposedly long ago
in the west for camp fires ] from...uh...cattle to compare to?

[exploration of sources and colors of such chips deleted]

At the risk of taking this seriously, wouldn't an empty stock car tend
to be clean and cleared of such debris? I'm thinking of Al Hoffman's
rather vivid description of the condition of poultry cars after they've
been off-loaded, and how quickly and thoroughly they were cleaned.
Shouldn't think stock cars would be all that different. So unless
you're going to model loaded stock cars, flat black would seem like the
best bet for a floor weight.

For some reason this brings to mind a Mason Williams' Smothers Brothers
sketch, which CBS censored:

Tom Smothers: In Russia, they've got this really great ballet....

Dick Smothers: Bolshoi.

Tom Smothers: No, really, they do!

Tom Madden