I'm an oldfag and I actually know the answer to this!
A long time ago, almost no gas stations in the US sold diesel. And no cars were sold in the US that burned it. There were a few trucks, but if you had one of those trucks, you had to go to a truck stop near an interstate to buy fuel - you couldn't buy it in town.
During this time, gas pumps could be whatever color. Also during this time, gas station companies tried to market themselves as having a different product than the guy right next door - even though that wasn't at all true, all the gas was exactly the same. They were trying to get brand loyalty. There were actually commercials on TV for gas stations.
…and, since gas pumps could be whatever color, the marketing people started coloring the pumps to match their advertising. So you'd go to a gas station and there'd be a black pump, a silver pump, and a gold pump - and they'd have brand names like "Texico Superfuel" (I can't remember any of the actual names, but they sounded like that).
In the 1990s, car makers started selling diesel cars in the US. Now that there was a market for diesel, gas stations started selling it. Unfortunately, most people are fucking idiots, so there was a huge problem of people putting diesel in their gas cars. The solution was that all the stations agreed to color diesel green. Why green? Simply because none of their marketing people had ever used green for any of the "brand name gas" that they had invented.
I'm 100% sure that's the reason. There's no law or industry standard forcing them to use green. It was just picked because it was an unused color.
So why is the gas price red? Well, what other color could you put on that sign that would be both distinct from the green and not overpower it? Yellow? Maybe. I think they just picked red because the diesel was green and they thought that would catch your eye.