Subpixel
Each pixel on an LCD or OLED screen is made of red, green and blue subpixels that mix to form its color. A fault in a single subpixel produces a colored dot rather than a black one.
In practice
Subpixels explain why a pixel fault is often colored. If the red subpixel of one pixel is stuck on, you see a red dot. If the green one has failed, that pixel looks magenta on white. Knowing which subpixel is affected helps you describe the fault precisely when you contact the seller or the manufacturer.
How to check
Step through full screen red, green and blue in the dead pixel test. A failed subpixel shows as a dark dot on its own color. A stuck subpixel shows as a bright dot on the other colors and on black. Looking closely with a magnifier or a phone camera zoomed in makes the subpixel structure visible.