Item descriptions:
A small, droplet-shaped bottle filled with a pink liquid. Upon close inspection, its stopper is blood-red and leaf-green.
A tiny, droplet-shaped bottle filled with a blue liquid. Upon close inspection, its stopper is blood-red and leaf-green.
Two bottles, one small and full of pink liquid, one very small and full of blue liquid. They fit neatly together. Upon close inspection, their stoppers are blood-red and leaf-green.
Item purpose:
Makes creatures instantly ready to breed. The pink bottle is for a female, the blue bottle is for a male. The bottle set is for both.
The pink and blue bottles are just used directly from inventory.
If you have the bottle set, you use it on one of your creatures, and then click "breed" for that creature. In the breeding selection menu, any creatures not yet ready to breed will be greyed out. If you click one of those, it will also be made ready to breed and will be bred with the first creature. If you don't click a greyed-out creature, you end up with either a pink or a blue bottle (whichever side wasn't used) in your inventory. The set is for convenience.