whiten, madden, soften
*bluen, *greenen, *yellowen, *iraten.
what conditions must ve met by the stems to /-en/ attach?
I'm not sure that there is a simple answer to this. It's mostly found among one-syllable words. As you have seen, colours, for example, is an error which is inconsistent. I am afraid that this is probably a case for dictionary use rather than rules.