An extensive visual preference study conducted with 274 Australian office workers found that any green roof is preferred over a concrete roof and taller, flowering, and/or grassy vegetation is preferred over lower-growing red succulent vegetation.
Lee, Kate E., Kathryn J. H. Williams, Leisa D. Sargent, Claire Farrell, and Nicholas S. Williams. (2014). Living roof preference is influenced by plant characteristics and diversity. Landscape and Urban Planning, 122, 152-159.