The image sensor in almost all digital cameras, with the exception of Sigma's, is a color filter array (CFA) sensor. The image sensor itself is monochrome; it detects light intensity, but not color. The CFA overlaying the sensor's light-sensitive photodiodes is a mosaic of red, green and blue (RGB) filters in a checkerboard-like grid. Therefore, each photosite, corresponding to a single pixel, receives just one of the three primary colors.
In this kind of system, a 15MP CFA sensor allots 7,500,000 pixels to green light and 3,750,000 each to red and blue light, respectively. Left as is, this checkerboard pattern would create a strange image, so a process called color interpolation is used to blend in neighboring pixel color information. For example, a green pixel gets color information from adjacent blue and red pixels, and so on.