Different phosphors which emit different colored lights are used to get a colored display in CRT. This can be achieved either via Beam penetration method or by Shadow mask method.
Beam penetration method: It is a cheaper method and is used in Vector scan displays. In this method the inside section of CRT is coated with red (outer layer) and green (inner layer) phosphors. If the electrons are slow they penetrate only the outer layer thus emitting red light, and if the electrons are moving fast they penetrate the outer layer and the inner layer. The electrons speed is also adjusted in such a way that by combination of red and green, orange and yellow color are also produced. The limitation of this method is that only four colors can be displayed in the screen. Since we have only four colors the quality of image is diminished.
Shadow Mask Method: It is used basically in Raster scan displays (color TV). In this method, the screen has 3 phosphor colored dots which emits colors red, green and blue in a single pixel position. We use 3 electron guns for emitting colors red, green and blue. Now, three electrons are fired from these guns simultaneously and when they pass through shadow mask they form a phosphor dot triangle of colors red, green and blue in screen. This triangle is actually a pixel in the screen. The color of this pixel can be controlled by controlling the intensity of the electron beams. Also, by controlling the intensity of electron beams we can produce other colors which are combinations of colors red, green and blue.