Having kept over 70 FW tanks at one time I agree there are some pretty colourful fish. Breeding is easy with most but a challenge with others. That being said. It's easy to breed if you're just going for quantity, if you are going for quality it takes a lot of work. Keeping genetic records and keeping lines for inbreeding and for out crossing makes for more work.
Here is a friends site. He studied guppy genetics for 4 years and at one point had the most popular Guppy site for serious professional breeders in the world. His lines sold for as much as $500US a trio.
http://www.wizardinteractive.com/guppyinfo/
Here is a link to a picture of a Half Black Pastel, one of my favorites.
http://www.geocities.com/rdarmani/et3.jpg
Here is some pictures of some guppies from a show breeder, remember these are not show quality fish but may have the genetics to create a show winner.
http://ppga.tripod.com/lukesales2b.html
As for Discus they aren't as difficult to keep as most people think, they are easy to breed as well, the tough part is raising the fry sometimes the parents just don't co operate. They don't need daily water changes, weekly is fine but daily will help them grow faster. Up here (Burnaby New West) our water is to soft out of the tap for Discus so we need to add a tiny bit of R/O Right or another remineralization formula. We need the hardness so their eggs don't turn to goop. With no hardness Discus eggs are so soft that most break before the embryo is developed enough to hatch.
Doug