There is no easy way of changing folder colors right from Opus, regarding the folder icon itself, except the standard routine (-> properties etc.), but you can easily set colors for the folder names by using Properties SETCOLOR=, what should meet your requirements. The colored folder names will appear in the folder tree.
You can define colors in -> options -> preferences -> display -> colors & fonts. You can use the "new" item there to create & set colors. You can set up shortcuts for quick changing of folder colors. See also this thread concerning setting folder colors via context menu: