Last I heard there was still disagreement whether alba and carthaginensis were separate species or just one with some variation.
Alba means white, and the flower is cream color to pale yellow, viridis means green, and the flower has a distinctively greenish cast, with the two plants side by side it should be obvious which is which, otherwise a subtle difference perhaps.
Tiny thorn-like projections under the leaf along the midrib is positive ID for Viridis, but these espinas are often lacking.
Another subtle difference, the leaf margins of Alba are quite wavy, the leaf margins of Viridis tent to be mostly flat with maybe a few waves.
There is also a morphological difference in the stipule, exactly what escapes me at the moment, but the most telling difference imo is the petiole, on Alba it is much more pronounced. See:
https://www.erowid.org/library/books_online/ayahuasca_apa/aya_gallery_psychotria_viridis.shtmlThere is some free reading at the site by Trout and others, iyi.
There was also an interesting video I had, I'll see if I can find that.