What colors do your parents show?

How much mom and dad resemble each other will with what the kids look like. If it is likely that there were many sires, predictions get more interesting.

If you want to separate fry out (darn, here we are spending money on more tanks or at least plastic ware), watch for the little bitty gravid spot on the little bitty females.

If your eyes are good enough to see the developing gonopodiums on the males, go for it. At least one guppy breeder has been frustrated by the (almost) clear colored male in his female tank which cheerfully inseminated that batch of virgins with his less than desirable color pattern. :(

Considering your question, I stopped by
http://www.ifga.org/_mainframe.htm
which includes a lot of articles from people who have been IFGA national champs at various times.

You may want to look at the colors of your guppies and then see what articles on breeding those colors says.

I wish there was one big guppy color chart - and there probably is - but I haven't found it.

The IFGA site does have a big chart of their color classes though.

Generally if the male and female is of the same strain, the males' tail and dorsal fin colors will be mirrored, in more modest intensity, in the females' fins.

If there are differences, you know the fry are going to be more diverse.

Take a look at this page:
http://www.ifga.org/_guppy_store/gs_white.htm

The blue and green pairs pretty closely resemble each other, Interestingly the reds are no as similar. But they should be consistant for the strain.

On this page http://www.fancyguppy.com/pictures/pictureframe.htm

some of the female half blacks are more black (or darkened) than others.

I would breed the darker females.

That is another thing too, When strains are crossed, some colors may be attached to the X or Y chromosome! Also while some colors carried in a chromosome will be dominant over another color in the other chromosome, some will have an incomplete dominance and some characteristics will be intermediate.

Now if a fish's characteristic is controled by genetic material on two chromosomes, then there will be more elaborate combinations between the four chromosomes of the parents.

Right about now, when I hear this stuff, I start making b-b-b-b-b-b-uh sounds.

It doesn't have to get any more sophisticated than you see a really cool male and a really promising female. You raise them separately from other prospective mates, in time you put them together and let nature take it's course and then your raise the fry by themselves.

Some show breeders let the first batch or two go and only save two batches of fry from a specific pair. Other wise they get overwhelmed by youngsters.

Nasty as it sounds, sometimes having a predatory fish in it's own tank to take care of the unwanted fry is useful.

If they aren't likely to get chilled, fry can be raised for a while in smaller containers - where they will actually see food, such as freshly hatched baby brine shrimp, more readily. (Be even more faithful with water changes there.)

It is good to select for size in prospective breeders too.

People breeding for guppy shows and - you will see this in those recommended articles - people crossing strains will really cull (select out) the fishes they don't want to breed.

Some of those culls get left at pet shops. Some are destroyed.