A fairly uncommon cultivar, 'Rosevallon' is generally only seen in botanical gardens with extensive Rhododendron collections or sold by specialist nurseries. If you were to walk by this plant in the garden, it's likely that you'd miss its most striking feature: the colour of the underside of its leaves. As you can see in the first image, the leaf undersides seem to simply be heavily shadowed and are easily missed. Bending its branches, however, reveals its secrets (I wish the second photo had a bit more depth. Ah well.).

The species Rhododendro neriiflorum is native to the Himalayas. For more on rhododendrons, I highly recommend the Rhododendron Species Foundation and Botanical Garden site.

Botany resource link: Flora of Taiwan online database. We have the print version at UBC, but it's good to see the information available online. If you find the information too technical, perhaps you'd enjoy some of the images: example 1, example 2 and example 3.