Our eyes are most sensitive to green light, so the green light is the most visible.
Everything higher or lower in frequency is less visible. Ultraviolet (higher than violet) and infrared (lower than red) are completely invisible.
Frequency of light affects many things. Low frequency waves require a bigger source to create, and a bigger receiver to receive. Materials behave differently in different light: glass is transparent for visible light, but not for ultraviolet. In x-ray, your skin and bones are transparent. And so on.
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