My take on the line is that Pete St John( Sean Dempsey)recognised that the world was changing and that generations in the not too distant future will all be a shade of brown. After all the song was written not too long after the segregation - desegregation clashes of the 60s in the US, and even today   mixed race marriages cause friction in certain societies

He wrote some cracking songs that seem more folk than folk.(at the risk of opening the hoary old argument about what is folk)