Mixing Caps are used to transfer paint from one can into another - either to make a new color, or to combine a few scrap cans together.
First you need a set of mixing caps, there are three types available, all three work as well as each other.
Next you need two cans of paint - decide which one you want to put paint into and take that can and put it into the freezer for at least half an hour. The idea behind this is to lower the aerosol gas pressure. Remember science class? The molecules in gas move further away from each other in a hot atmosphere, and move closer together in a cold atmosphere. Basically chilling the can makes the paint easier to mix with as the pressure is less powerful and the gas pushes it out slower.
Once the first can has cooled get the other can and place it in a bowl of warm water (NOT boiling water!). This increases the pressure of the gas that will be going to the chilled can. The fact that the pressure has increased will make the paint flow faster in to the can that's cold. So because of the pressure differences the warm can 'overpowers' the cold can and forces paint into the cold can.
Now connect the two cans together with your mixing caps - the warm can should be on the bottom and the cold can upside down. Once both cans are connected to the mixer caps push the nozzles in together at the same time. You should hear a 'sloshing' sound if you put your ear to the can - this means that the paint is flowing. After a while both cans will return to room temperature and the mixing process will slow down. When this happens give both cans a shake.
After you have mixed both cans disconnect the mixer caps and shake the cold can for at least two minutes then give a test spray. At first the paint will be the color of the warm can but it will go to the true color after a few sprays. This is caused by paint still in the valve of the can.