A large part of vision underwater is being able to distinguish different colors. Seeing colors underwater depends on the amount of light reaching the particular depth at which one is at. Another factor in seeing underwater is the condition of the waters and, more specifically, the conditions of the surface. There are several ways to make the colors easier to see. However, it is most important, to simply understand that colors change underwater and it is sometimes hard to distinguish between them.
One factor in seeing underwater is the fact that as light passes through the water it is absorbed, and much of it is lost in the process. This causes objects to lose their color as they go deeper down or further away. To add to this, the wavelengths that make up our perception of color are absorbed differently. The length the wavelength changes how fast the color is absorbed. Red has the longest wavelength, more than 700 nm. One "nm" stands for one nanometer, which is on millionth of a meter. After red comes orange which is somewhere in-between 700nm and 600nm. After orange comes yellow and so on, all the way down to the blues and purples which are the shortest at around 400 nm.
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Depending on the length of the color's wavelengths you can predict how a color will change underwater. For example, in clear water, the longest wavelength is lost first. So if you were in a pool swimming downward, the first color that would be hard to see is red.
Another factor in seeing color underwater, is the condition of the water. Light from the sun is reflected by the surface of the water. This means that the surface of the water can cause significant change in one's perception of color underwater. Different surfaces can be different amounts of bubbles, pollution, decomposing plants or plankton. Even if the change is simply more motion in the water, causing more bubbles and a different angle between the rays and the surface of the water, light would be absorbed faster, and color would be therefore lost faster. However, something such as plankton can significantly change perception of color underwater. This is because plankton absorbs violets and blues. So the presence of plankton would cause blue and violet objects to lose their colors much faster compared to red and yellow objects. Red was the first to lose its color in clear water, and the blues and violets were the last. So the condition of the water can ultimately reverse the situations, before the longer wavelengths were the first to be absorbed and with plankton the shorter wavelengths are the first. Thus, the condition of the water is a huge importance when seeing colors underwater.
Understanding how colors change at different depths and in different conditions is the first step, understanding what they change to and how to work with that is the next. In clear water, if you go down far enough a red object either appears unlighted or black. This makes since as clear water absorbs red light and eventually you can reach a depth where no red light reaches the object. The same thing could happen to a blue object in coastal waters, it could appear black. Even though red is absorbed faster in clear water and blue in coastal waters, all the colors are absorbed in water, just at different rates. So the farther you go down the less color is perceived. Plus, the further down you go different color objects all start to look the same color, the color they all look like tends to be the color that is best perceived in that water condition. For instance, if it was clear water, at a certain depth all the objects would start to look gray. So even before you reach a depth deep enough to make colors look black you can get easily confused between the different colors. One way to distinguish between different colors is their relative brightness or darkness. Several of the most visible colors are light, bright colors that cause a good brightness contrast with the dark water background. If there was a different background, such as white sand, darker colors would be easier to see. Another good way to distinguish different colors is to use two colors that cannot be mixed up in any type of water. A good example would be orange and green.
