Nearly white

Assuming you're asking about Trailer Train flush deck pig flats, they mixed sand into a very light grey.  So fresh out of the shops the deck looked almost white.  Sometimes the whole deck got the white coating, otherwise only the outer thirds where the tires rolled (or sat) got it.

"Standard deck" pig flats (side sills higher than the deck) generally had sand mixed into the body color (red-brown until 1971, then yellow), but there were some with light grey decks.

EDIT:  I found one pic from 1975 that shows several different classes of yellow TTX cars and all have light grey anti-skid on the outer thirds of the deck.  So from this, which is before my time in railroading, I must infer that Trailer Train was using light grey in the early years of yellow cars and either they gave up by the '80s or the cars I remember were all too dirty to see the light grey.

So the answer is there is no one answer.  And whatever color you choose, it didn't stay clean for long.  And then the areas where the trailer wheels sat tended to get rubbed down to bare metal.

I'll see if I have any useful pics handy.

Scott Chatfield, 

Formerly of Southern Railway's Intermodal Department