According to the documentation: "Contrary to TextEdit, DEVONthink allows you to set the background colour of selected text. Simply select the piece of text you want to highlight, choose "Tools > Show Colors", check the option box "Background" and then select the colour of your choice."
Either I’m misunderstanding something, or this is currently (DTpe 1.6a) not working as advertised - the background of the entire window is coloured, rather than just the selection.
Very useful feature though, I look forward to it being fixed.