An achromat can bring two colors to a common focus. The conventional approach is to bring the red and blue to that common focus. You see that in spot diagrams that has green as the focal point. The red and blue out of focus color blurs are typically much larger than the airy disk but are of equal size. That out of focus red and blue is the purple "CA" border around stuff you typically see in achromats.
My take on the Istar R series lenses is that the "reduction" in CA is more of a "redistribution" than an actual reduction. Just take a look at how the spot diagrams are presented. A good, clear example is the 6" F8 R35.
The designs appears to bring the focus of the blue portion of the spectrum much closer to green/yellow than that of red. As presented, the on-axis spots for the green and blue fill the airy disk but the red is puffed much further out. You also see this in the contrast plot for the different colors, where the green and blue are grouped together and the red falls well below them.
Now take a look at the same spot diagram for the 6" F5 achromat. You see that the red and blue spots are much closer to each other in size, which is very much like a conventional Fraunhofer achromat design.
So to me, it seems that the two colors that Istar has chosen to bring to a more common focus in the R series lenses, are the blue and green/yellow, letting the red fall off.
From a visual point of view, pulling blue much closer to the green/yellow focus and letting the red focus bloom, may make sense. In dimmer light, the eye/brain is much more sensitive to blue than red. Puffing the red focus out even more will make it much dimmer and much more difficult for the eye to detect since it's spread out much more. With the blue also now being much closer to green, it seems to give a more white image on some objects as some have reported.