The short explanation (assuming old CRT TV's): A TV shoots a beam of electrons at the screen, and the screen is coated with dots of material (phosphors) that light up in red green or blue colors. The beam is scanned left-to- right and up-to-down rapidly by electromagnets. The human eye mainly perceives red, green, and blue colors, so by controlling the amount of these three colors on the screen pretty much any color can be generated. Because they are so small you don't see the individual red/green/blue dots, they just blend together into smooth colors in your eyes. The radio signal contains information about the color and brightness of each dot (or pixel) on the screen so the screen displays a rapidly changing picture.

More modern LCD TVs are similar but instead of an electron beam and phosphores here is a bright white backlight (white contains all colors), and using polarization, liquid crystals inside each pixel allow certain amounts of red, green or blue to be transmitted to your eye from each pixel. Plasma TVs use a tiny red, green, or blue flourescent bulb inside each pixel. Some cell phone screens now use tiny LEDs inside each pixel.