Color has nothing to do with the sulfur content. As mentioned, just a dye for tax purposes.
I believe, this argument is pretty much moot as of June 2010. In June 2010 the EPA mandate limited sulfur content of diesel to 15 ppm, for both on-road and off-road, amber or red. The only exception was NRLM, non road locomotive or marine diesel, which is allowed 500ppm sulfur until sometime in 2012. Most of us wouldn't have access to NRLM fuel which means we are using 15ppm max sulfur, ULSD, whether red or amber. NRLM is not the red, off-road, farm use diesel most of us are familiar with.
It is interesting to note the pump lableling requirements did not keep up with the EPA mandates. There are pumps here in IN that still read high sulfur greater than 500ppm, but it is 15ppm max sulfur because that is all the retailer can buy from the refiner. Don't trust the pump, talk to the supplier if it is important to you.
Wxman