we all know about the VIBGYOR the seven colors these colors are shown here. All objects absorb some colors and transmit some colors for an example apple looks like red is that the apple absorbs all the colors except red the red is transmitted that shows apple as red.
another example
we know the visible light range from 400 to 700 nm here red have the long wavelength that gives the ability to that travel from a large distance and it can able to seen by the observer even he stands the huge distance from an object. that is the reason people use red color in all kind of caution boards like a signal.