Limia and Gambusia are live bearers native to Cuba, as others have said, the type dither is not as important as that you have them to make your cubans comfortable. Cichlids watch schooling fish or ditherers to gauge their safety.
2 cichlids alone in a very white biotope will feel vulnerable and not show true colors. Hard to tell male from female in that situation, because they are so washed out. I use filter sand, but use dark logs and floating plants to make the cichlids feel safe.