Coloretto vs. Aquaretto and Zooloretoo, what are the differences and how can I tell which to buy?
Coloretto is MASSIVELY different to Zooloretto and Aquaretto.  To be more specific: Coloretto is the mechanic on which the other two games are based, but to compare them is a bit like comparing, say, addition to mathematics.

In Coloretto, your move is either to play a card onto a pile, or pick up a pile.  Once you've done that long enough, you look at everything that all players have picked up, and then score.  In Zooloretto and Aquaretto, the piles have become "trucks", and the cards have become animals, or fish.  You still have the same basic mechanic - either draw and play an animal onto a truck, or take a truck - but collecting animals is no longer the sole object.  Once you take a truck you have to place the animals in various parts of your zoo.  There are placement restrictions, bonuses for filling areas, the chance to get extra free animals by placing breeding pairs in the same enclosure.  Players can, instead of taking their normal turns, pay to buy animals from each other, swap animals between enclosures, discard excess animals.

As you can see, just because the central idea of Coloretto is present and correct in all three of these Michael Schacht games, comparing them is like comparing milk and cheese!  I like all of them, but Coloretto is good when you want a quick, abstract game, and the others for when you want a moderately (but not very) complex game... with cute baby pandas.  (I think it's really the cute baby pandas, much more than the mechanic, that made Zooloretto a hit in my gaming group!)

ETA: One final point.  You might assume that Zooloretto and Aquaretto are fundamentally the same game but with mammals instead of fish, but they really aren't.  They have extremely different enclosure rules, animal behaviour rules, and ways of scoring points off specific animal configurations.  Essentially they are the Coloretto mechanic taken away and used in two different boardgames.  But they are of a similar complexity level.  I felt Aquaretto was a little bit more complicated, but maybe that's just because I've played Zooloretto a lot more.  I'm told that it's possible to combine Zooloretto and Aquaretto into one mega-zoo game, which would be intimidating but definitely fun.  Our group hasn't tried it yet, but it can only be a matter of time!