Same may know I had beed trying to determine the difference (if any) between AR-303 Rosso Corso and AR-501 Rosso Classico.
PPG uses different codes for these, perhaps indicating they are different. PPG calls AR-501 > PPG-71718. And AR303 > PPG-71297.
I have finally found color chips of both colors and a bunch of other data. I found only one paint chip for AR-303, but I found several for AR-501. Both of these chips were very similar, essentially the same! Not surprisingly there were small (insignificant to me) differences among the AR-501 paint chips from different years. I noticed that the formulations for the color also changed slightly from one year to the next in at least the Glasso paint book. Unfortunately, there was no formulation given for 303 so I could not completely compare it to 501, but again, it looked very similar, the same to me.
I also found a Dupont book containing both colors. The dupont book lists the same code for both 303 and 501! So this is more evidence the colors are the same.
You may download a boatload of colorcharts here from my website, if you like. It includes many of the other Alfa colors as well.
One pic below shows the AR 303 and 501 chips side by side, both from Glasso. The other pic shows the Glasso AR303 chip behind another unknown company's AR501 chip. The unknown company chip has a round hole it so the AR303 shows through.
Both pics show the color being very similar. Obviously the colors are distorted in the photos, but you can see the similarity. I now consider the colors the same.