Fluorescent—or "neon"—colors do that, too, but they have an additional trick. Fluorescent colors also absorb and convert ultraviolet rays, which are invisible, and convert them into visible light. Fluorescence shifts energy in the incident illumination from shorter wavelengths to longer (such as blue to yellow) and thus can make the fluorescent color appear brighter (more saturated or lighter in luminance) than it could possibly be by reflection alone. The absorbed energy excites electrons in the pigment molecules to a higher energy level, which then relax back to their ground state by emitting light at a longer wavelength than that absorbed, resulting in a visible glow
As a result, your eye perceives a far more saturated color or a tone that's higher in tone relative to the white paper they're drawn or painted on.
Ultraviolet light is usually present in outdoor light, whether direct sunlight or overcast. Without a source of short-wavelength light (like a black light), the fluorescent pigments won't stand out. As soon as you add an ultraviolet light source, the fluorescent pigments will appear to glow, while conventional colors remain dull and hardly visible. If a subject is lit only by ultraviolet light and no visible wavelengths, fluorescent colors will appear to glow magically in the dark.
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