The brightest colour of light is green (a green-yellow variation being the best, to be exact)! This can be seen easily with lasers, the green ones are usually bright enough to see the beam all the way down to the <5mw versions. I’m sure a yellow laser, provided the yellow light was yellow enough to see would also show a beam. The yellow lasers do exist, but because of the rarity, the cost is higher.
In paint, yellow is the most showy colour, as it stands out the most. It’s so in your face it is also known as the most irritating colour. One of the reasons it’s used for road signs is that you can’t help but notice it.
Back to light, red is the color that can be seen the farthest. Red light has the highest wavelength (of visible light) thus is blocked by the least obstructions, for example; dust or atmosphere. This also explains why a sunset has a red appearance.
Light is composed of waves, like sound waves, microwaves and x-rays. A wave can be best and most simply visualized as a line that alternate between up and down in a smooth curve. The wave is usually described in a length, the length is determined by measuring the distance between the peaks (the space between the highest point in the wave). We just found out that red light is the most penetrating of the visible light, what about some other examples?
Radio waves, being even further apart at the peaks, are blocked by far less, including walls. This lets you listen to your favorite tunes inside, even through concrete walls. X-rays waves however, are tightly packed and do not easily travel through materials. To take an x-ray, it takes lots of energy to get them to travel through our flesh, but they are easily blocked by our solid bones. Microwaves lie somewhere between x-rays and radio waves. With enough power/energy, they can penetrate soft masses, the efforts are enough to excite water molecules and heat your food, but not the dish. The dish can get hot by being next to the hot food though.
So, why don’t x-rays or radio waves harm you like microwaves can? The answer to this is that the really high frequencies of radio waves and really low frequencies of x-rays don’t react with the water in our bodies. Unlike microwaves are tuned to specifically excite water molecules on just the right frequency.
Ok sure, you say, that explains why the sunset is red, but why is the sky blue or black the rest of the time it’s not red? Well, to start, we know black is the absence of light, when the sun creates a shadow from lighting the other side of the earth, it goes dark. As for blue, you know that red is on the higher end of the wavelengths, well, blue is on the lower end. When all the other wavelengths of light have been scattered in our atmosphere, all that’s left are the slower ones, blue is the most prominent to remain. Now violet is the lowest visible colour, being just before the invisible UV (Ultra Violet), but it’s harder to see than blue, so our eyes just see the blue.
Sure, that sounds reasonable.. so why isn’t the sky green before it goes from blue to red, green is in the middle right? I blame the aliens. The actual reason is colour temperature. Next time you get an LED lightbulb, look for a ‘temperature’ scale on the box, it should start at blue, daylight should be on that end at around 6500k. The other end of this scale is reddish, with incandescent or candle at 2000k. Finally pure white light is in the middle at around 4500k. We aren’t cycling through all the frequencies of light, the sun only emits white light, we are just getting a varied brightness of white light and that’s how our eyes see it.