A light that can produce a range of colours based on the three basic colours red, blue and green. RGB lights are usually based on LED technology and available in different forms such as bulbs or strips. These lights are available for various prices depending on the type and brand.
Any lighting system that can produce many colours based on the three basic colours (red, green and blue) are known as RGB lights. One can find them in many various forms and they are almost always based on LED technology.
The other colours are produced by altering the power on the three different channels to create the needed mix. For instance, for purple only red and blue are used with a bit more blue but the green would be redundant and is thus kept off.
There are RGB lights that are in the shape of a bulb, a strip, a ceiling panel and a lot more. There are versions that are smart and can be controlled over the WiFi using one's smartphone.
The price of an RGB light depends entirely on the kind it is and the brand that made it.