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

A hybrid of H. rupifraga x H. plantaginea, this cultivar was registered by Jim Dishon of Michigan on 1994. The medium size plant grows to around 13 inches high with dark green leaves. Fragrant, lavender flowers bloom from mid-July into September.

Hosta Helper will call this plant 'Forever Green' (Dishon)
 

According to The Hostapedia by Mark Zilis (2009), a green form of H. 'Opipara' from an unknown originator was available by this name prior to the registration of the Jim Dishon plant.

Hosta Helper will call this plant 'Forever Green' (Hostapedia)