You would have to dye them Pullman Brown.
As a Necro look what I found:
Soderstrom is responsible for the famous UPS Brown as the company’s standard color. With the first Model T Ford painted
red, it stood out. However, an advertising man told Jim Casey that yellow was the most conspicuous color, so they painted
the second car yellow as they wanted to be conspicuous.
They had different schools of thought when it came to painting the third car, Jim Casey recalled. If they painted
it a
third color, perhaps the public might think they had a great many more vehicles than they actually had.
The other idea was to paint them all the same color to create a standard fleet. After much discussion, Casey
remembered, they decided to adopt the conspicuous yellow as the standard color for the fleet.
Soderstrom knew how the department stores would react to the bright yellow fleet and was appalled
when he learned of that decision. He explained that the department stores saw their own vehicles as a form
of advertising. For the stores to disband their own fleets and turn their parcels over to a company
like Merchants Parcel Delivery, they would want the change to be subtle and scarcely noticed.
He argued for a much more conservative color. The other partners, once empowered with that new
viewpoint, agreed.
Soderstrom, in looking for the proper conservative color, discussed the dilemma with an old
carriage painter named Charlie Place. Place told him about the recent experiments run by a railroad
sleeper-car company named Pullman. Pullman wanted their rail cars to look as clean as possible even
when out on the tracks subject to the elements of rain, snow and dust.
Their experiments resulted in Pullman Brown, a distinctive color that blended well with the dirt.
So the first Merchants Parcel Delivery fleet was painted Pullman Brown. During the early
years the exact color changed slightly to become the UPS Brown in use today.
Read more:
http://www.browncafe.com/forum/f6/why-brown-95074/#ixzz25E4UFvgR