I've personally never heard of an Endler/guppy hybrid being sterile. I have seen all kinds of Endler mutations for sell on Aquabid.com. Some of the color varieties couldn't be reached without crossing with guppies. The two color morphs that I raised were both from reputable people, and were stated to be 'pure' strains with no guppy crossing. I'm guessing it's possible that different isolated groups of these fish could vary in color?