Well Dandries,
Overhere it really depends on the kind of strain of endlers wether they'll turn colour faster or not in comparison to guppies.
And Istrom,
Your quote : I'm sure it takes a while, and even if you do get a magnificent strain there's no guarantee that it will be popular at all.
True! But a serious breeder will make a distinct choice how he/she will create a new strain without considering if the new strain will become popular as being a main focus.
I myself have some own created strains of endlers as well. But I've created them because I wanted to create own strains for myself without the goal of making them popular.
At this point one of my projects is a white endler strain. I've started this project last fall. And the whites will be presented during the international endler championship in Berlin, Germany this coming fall.
Here a sample of one of the males: