Oh, the pressure... actually as I'm trying to remember all the issues, I'm not sure I will get all the details right... if anyone wants to jump in and correct me, please do!
As most know, railroads will not provide specialized equipment. So customers must either buy their own or lease them from companies like GATX, ACF, Union Tank (UTLX), etc. In order to offset the cost of the equipment, a national mileage credit system was established back in the days of Jesus so that private car companies would get $0.xx cents per loaded mile, the credit amount being based on the value of the car. This system exists to this day. The cost is all worked into the freight rate, so when a clay company sells a paper company a car of clay, they add to the bill the freight amount and the paper company ends up footing the bill for the car costs. This system is a pain in the butt from an accounting standpoint, and the majority of shippers have gone to "0" mileage rates, which simply means that the mileage credit is backed out of the rate... lower rate, but no car compensation. ok....
Clay companies run the wheels off of their equipment compared to the chemical industry... most tanks only turn about 10 times/yr (plastics covered hoppers only cycle about four times pre year) As such, they were actually making money on the mileage credits, i.e., taking in more in credits than the monthly car lease cost. The railroads didn't appreciate this and I believe it was the ICC at the time (the mid 80s?) ruled that car owners/lessees could not be compensated with car mileage at a rate more than their equipment cost or some such thing. There were also caps issued on the mileage cerdit amount (it now ranges from ~$0.50-$.90+ cents per mile) Paint is part of the total car cost, so the clay companies decided to spend some of their "windfall" on fancy paint jobs rather than lose it.
As you will notice, most companies are content to leave their tank cars black with lessee stenciling. The reasons: Cost... and the only free advertising you get with a tank car is when the local news has its camera on it during a hazmat emergency!