Looking pretty is probably top of the list but there is also:

• hazard labelling
• visibility (i.e. don’t buy green-handled garden tools, unless you want to lose them every time you put them down)
• clearly showing where the buttons are, or marking different sections of an object
• helping to locate different, very similar things (imagine if all your USB sticks were the same colour)
• cost of different coloured materials
• individual identity of object “my drill is turquoise, your drill is yellow”
• to warn off predators