I actually just sent out my Save The Dates and put our website on the cards. Then on our website, we have a tab that says, "What to wear to wedding," and it explains that barefeet are welcome, etc......and to please wear shades of champaigne, white, or beige (but please leave the white dress for the bride) ;-)
I had to tell them now because most people want to do their shopping ahead of time for Jamaica....i.e., they won't be able to buy bathing suits or clothes in these colors in November! I just think it's nice to give a heads up....while being nice but straight forward.