Why is the color blue so uncommon in animals and plants?
The color blue is relatively uncommon in animals and plants due to the way in which pigments and structural colors are produced. There are a few reasons for this:

1. Absence of blue pigments: Many colors in plants and animals are derived from pigments, which are chemical compounds that absorb certain wavelengths of light and reflect others. Blue pigments are relatively rare in nature compared to other pigments, such as chlorophyll (green) in plants and melanin (brown/black) in animals. This is because the biochemical pathways to create blue pigments are more complex and less common.

2. Structural coloration: In some cases, blue coloration in animals and plants is not derived from pigments but from nanostructures that scatter light to create a blue appearance. This phenomenon, called structural coloration, is seen in the vibrant blue feathers of birds like the blue jay or the morpho butterfly's iridescent wings. Structural coloration is more complex and less common than pigment-based coloration.

3. Functionality and adaptation: The colors that animals and plants display often serve a purpose, such as camouflage, mating displays, or warning signals. Blue is less common in part because it is less functional for these purposes in many environments. For example, green coloration is more useful for plants, as it allows them to photosynthesize more efficiently, and it provides better camouflage for animals in forested environments.

4. Evolutionary constraints: Evolutionary history can also play a role in the rarity of blue coloration. For example, the ancestors of many animal and plant species may not have possessed the genetic traits or biochemical pathways necessary for producing blue pigments or structures. As a result, their descendants are less likely to develop these traits.

Despite its relative rarity, blue does appear in some animals and plants through a combination of pigments, structural coloration, or both. The unique beauty of blue coloration in nature can be attributed to the complex factors that contribute to its production.
If the color in animals and plants often serves  a purpose such as camouflage, then the color blue should be very common in sea animals?
Yes, the color blue is quite common among marine animals because it often serves a purpose, such as camouflage. In the ocean, blue light penetrates the water more efficiently than other colors, making it the dominant color in deeper waters. As a result, animals that are blue or have blue patterns can better blend in with their surroundings, which helps them avoid predators or sneak up on prey.

Some examples of blue sea animals include:

1. Blue tang: A brightly-colored blue fish that is common in coral reefs.
2. Blue shark: A species of shark that has a blueish tint to its skin, which helps it blend in with the surrounding water.
3. Blue whale: The largest animal on Earth, the blue whale has a blueish-grey color that helps it camouflage in deep water.
4. Blue-ringed octopus: This small octopus has iridescent blue rings on its body, which serve as a warning to potential predators of its venomous bite.
5. Blue chromis: A small, bright blue fish that inhabits coral reefs.
In addition to marine animals, some aquatic plants and algae also have blue pigments. For example, blue-green algae, also known as cyanobacteria, contain a blue pigment called phycocyanin.

It's important to note that not all sea animals are blue, as other colors and patterns can also provide effective camouflage or serve other purposes, depending on the specific environment and needs of the species.