Fluorescent lamp with and without phosphor coating
When an electric current is passed through the mercury vapor that is present inside the Fluorescent lamp, it excites it and produces short-wave ultraviolet light (LEFT).
But a phosphor coating on the inside of the lamp makes it glow white (RIGHT). The type of phosphor coating dictates the color of lamp
For example, in the tube above
a coating of red phosphor is applied the right, followed by a band of green, a
band of blue, then with all three mixed together to produce white light,
and finally on the left an uncoated region showing the mercury
And that’s how we get Fluorescent lamps of different colors!