Two of these trees are at some sort of county works yard near the traffic circle on Poipu Road on Kauai. You can actually see the trees and a very similar but not identical boat in google street view, except that I can't seem to link to the street view (you can get into street view after clicking on the fire hall, which is in the same complex). I'm wondering if they're Cassia × nealiae 'Wilhelmina Tenney'? Flowering time is late - that's supposed to flower April-September; this was November. I'm not sure if the photo 3rd to last belongs to the first or second tree. The second tree seemed to be properly finished blooming, if it's this cultivar. It had a few blossoms that looked similar to me to the ones on the other tree.
Cassia fistula x C. javanica perhaps? http://www.richardlyonsnursery.com/?p=240 Oops, didn't know it's been given the moniker 'nealiae'.
OK, thanks. I'll give up on the cultivar. I don't know if there are lots of them or just one other I've found that's white. [Edited]Now that I reread the second link below, it seems that the varicolored trees are Cassia x nealiae, and the pink and yellow ones are C. x nealiae 'Wilhelmina Tenney'. I guess the common name "Rainbow Shower" refers specifically to this hybrid. According to NTBG, whose photo looks not much like this, "n 1965, the rainbow shower tree become the official tree of Honolulu. (Neal,M.C. 1965. In Gardens of Hawaii.) (Staff of the Liberty Hyde Bailey Hortorium. 1976. Hortus Third.)" That's Marie Neal's name on the hybrid? According to Missouri Botanical Garden, the cultivar 'Wilhelmina Tenney' is what was designated the official tree of Honolulu.