Pigment PR102 is Natural Red Iron Oxide while PR101 is Synthetic Iron Oxide Red.

These are the four colours from Daniel Smith that are made with pigment PR102, aka natural red iron oxide. Interestingly, PR101 (synthetic red iron oxide red) can be made into eight different colours. These four colours together with the eight PR101 are red earth colours that mix well with blues to produce beautiful grays.

These are mixes with French Ultramarine. I've also included the common French Ultramarine and Burnt Sienna mix for comparison. These mixes looks quite similar. The mix that's slightly different is with Burnt Yellow Ochre which is slightly more reddish, and the resulting mix has a more violet hue to it. Other than that, it's difficult to differentiate these mixes.