They will go on adventures until they get lonely. When lonely they will group with their own kind and breed until they feel like an adventure again. When they breed they will produce a child that is a blend of the parent's colors. As time goes on they will slowly age and get weaker until they eventually die. They will also fight each other until there is just only type one left. Age, energy, and how many adventure's they've been on help determain their strenght when fighting.

You can roll your mouse over one to get it's status information. Yellow lines goto kills, green lines goto children, magenta lines goto mates, cyan lines goto parents, and a red line goes to who killed it. You can also click and drag the status box around the screen.

Things To Add

Having them eat each other would be a good way to clean up the corpses. It could also create more fighting among them which might be bad. It will be intresting to see what would happen when only one type is left and they start killing each other for food instead of breeding.
Have them get depressed when their social stats get really low (like -1000). Maybe make them go homicidal and kill others of their own kind when this happens.
When they are weak they try to avoid getting into fights so they don't die.
Have counters that keep track of population information.
I'm not sure if I want to add walls around the edges of the world or not. It's no fun when a whole bunch of them go wandering off the screen for a long time, but I like the idea of them having freedom too. Another thing I could do is have them appear on the other side of the world when they go off the edge, but this could cause a lot of problems when they group togeather.