RGB LEDs are widely used in many of the projects with Arduino platform. They work on the basic concept of combination of the basic colors of any shade, that is, red, green and blue. All the colors have these elementary color components in them. Thus, we can make any color we want using a RGB LED.
In the market, there are two types of RGB LEDs - common cathode and common anode LEDs. People (especially beginners) often get confused as common cathode ones are easy to use, but the common anode one behaves differently, even if they are from the same brand and look the same externally on same Arduino sketch.