TFT RGB LCD construction and
An LCD doesn't emit light.  It either has a reflector behind it, or a separate light source (backlight).  The backlight may be controllable on any scale from a monolithic whole to a pixel.

The LCD selectively blocks light.  In a color LCD, each pixel will have 3 segments which correspond to red, blue, and green filters.  Color is achieved by blocking these filters to varying degrees with the corresponding element of the LCD matrix.