This is one of those cases where there are two hostas with the same name. One is registered and, therefore, the "official" version of H. 'Royal Rainbow' while the other is non-registered plant and needs to be renamed to avoid further confusion.

This hybrid of H. 'Bold One' × H. ''Flamboyant'' was registered by Paul Aden of New York in 1978. It forms a medium size plant of green foliage that has yellow streaked variegation.

According to The Hostapedia by Mark Zilis (2009), "...probably no longer exists."

It is a sibling to H. 'Look See'.

Hosta Helper will call this plant 'Royal Rainbow' (Aden)

According to The Hostapedia by Mark Zilis (2009), a different plant has been sold as 'Royal Rainbow'. It is a light green hosta with marginal variegation that changes from creamy yellow to white. This plant is similar to 'Shade Fanfare' and is definitely not the plant that Paul Aden registered.

Hosta Helper will call this plant 'Royal Rainbow' (Hostapedia)