It is a slot for things like Graphics cards and sound cards to fit in, its faster than a normal expansion slot and usually a motherboard will have 2-3.
Its for expanding that computer's ability. You can add stuff like a tv tuners, sound cards, network card, wireless networking, more ports, video cards, more sata or IDE controllers, etc. Mind you...
There is no special color requirement for PCI slots. In most motherboards, the connector is white, but some have them in a variety of colors, such as blue, brown, or orange.