I couldn't find Andre kaplan (official maintainer) email so I post her my contribution. I modified a little bit th whitespace plugin in order to display colors in indentation to highlight code structure. You can find the modified version of the plugin and a screenshot here :

http://www.limsi.fr/Individu/vernier/IUP/

does someone find it usefull (code readability probably)?
do you know previous work on the topic ?
do you have any idea on which color for a loop or for a "if-then-else" statement ?

fred