This technique is used in raster scan display devices. It gives more colors than a beam penetration method.
A shadow Mask CRT has three phosphor color dots at each pixel location (red light, green light and blue light.
This type of CRT also has three electron guns one for each color dot.
A shadow mask grid is installed just behind the phosphor coated screen.
When the three beams pass through a hole in the shadow mask, they activate a dot triangle, which appears as a small color spot on the screen.
More than 17 million different colors can be obtained in a full color system.