Red is still the least apparent color in daylight, but it may appear more visible than other colors due to contrast (red is not as common in nature as some other colors and may stand out better for this reason). It is especially noticeable in light or yellowish green. Blue is the opposite - it is very visible against the background black of space or the ocean, but it is less apparent in general daylight.
In darkness, red is again the least detectable color, followed by blue then green. Yellow is the most detectable color because it is closest in wavelength to white light from a bulb or star.
The human eye has limited sensitivity to color. At any given moment, you can only detect objects that are big enough to see with your own two eyes. Smaller objects can be seen with greater clarity in other colors. For example, if you look into the sky on a clear night with the naked eye, you should be able to see stars beyond the constellations you are used to. If you do not know which stars are which, you could use blue and green filters to reveal hidden stars.
In conclusion, visible color depends on both object and environment. Red is least detectable color in daylight because it is not as common an object as blue and green. In darkness, red is the least detectable color because it is next to white in wavelength and so it stands out from other colors which follow blue and green.
Red, on the other hand, is less apparent from a distance. In low-light circumstances, rods in the eye help humans see, and yellow is the most apparent hue from a distance in darkness. 6 out of 10 people can only detect red colors at close range.
Furthermore, red is one of the most difficult colors to see clearly against white or black backgrounds. Humans have two types of color-sensitive cells in their retinae, called cones. These are sensitive to different wavelengths of light: short, bright colors like blue and green trigger the response of both types of cone. Red shifts into the visible spectrum where it becomes more apparent but does not need as much energy to detect. Cone cells are particularly sensitive to red lights because they use the information processed by these cells to colorize our perception of reality. For example, when driving at night without using headlamps, our vision of the road ahead is colored red due to the presence of nearby traffic signs.
Finally, red is the most common color for blood, so it plays an important role in biology. The word "red" comes from the Latin rhedos meaning "blood", and this is exactly what this color represents. The human body uses red chemicals to make healthy muscles strong and healthy veins weak.
Green light is the easiest for human eyes to detect throughout the day, followed by yellow and blue. This is one of the reasons why traffic lights are green. Red is also employed in traffic signals because it stands out against all the green in nature, despite the fact that it is the least apparent hue from a distance.
In reality, the colors of the rainbow exist in almost every shade between red and green. We just happen not to be able to see them all at once. Instead, our brains combine these near-identical colors into one uniform tone, which is why we perceive red as being more intense than green or blue.
This same principle applies to objects that are not visible to humans, such as infrared sensors used by security cameras. These devices can tell the difference between people based on their unique patterns of heat absorption across the spectrum from red to green. They simply cannot see color itself; instead, they "see" the relative intensity of heat coming from different parts of your body.
For an object to be considered white, it must reflect equal amounts of red, green and blue light. The term "white" comes from the fact that blackboards are often painted with this color mixture to make its otherwise opaque surface easier to see.
The color of sunlight varies over time as well as from place to place. During daytime, the sky is mostly blue with some clouds here and there.