Why are bright colors eye-catching?

Bright colors can give people a pleasant feeling as many of them are mood enhancers. 

Bright colors can have a nostalgic effect on people as they can remind them of the colorful things they had in their childhood. Candy Crush, for example, is a game that affects people's mood in a positive way by using bright colors that resemble the colors of candy. (See Why do kids love candy so much?)

The right combination of bright colors can increase the metabolic rate of a person, make them more energetic and make them generally feel better about themselves.

Colors such as grey, brown, and black can have a bad effect on the mood if overused. The eyes of humans usually go to the bright colors first because those colors make them feel better.

Some psychologists say that we see bright colors first because they remind us of the fruits and healthy foods we need to eat. The human mind has a special mechanism to recognize those colors because they can help the human survive.

In another theory it's suggested that we pay attention to bright colors first because of their strong emotional effect on us. (See 20 Signs That You Are Emotionally Sensitive)

Evolutionary psychologists suggest that humans learned to spot bright colors because poisonous food had bright colors and so the ability to recognize bright colors fast helps humans to survive.

Some people suggest that bright colors get noticed first because their wavelengths get noticed by the brain more than dark colors.

Many people suggest that bright colors catch our attention because most of the other elements in a specific setting are usually dark. Those people suggest that the colors themselves aren't catchy but it's the contrast that makes them stand out.

In their early years children are drawn to bright colors because their vision, which is not yet fully developed, allows them to see those colors easily.

Some people suggest that we were conditioned to notice some bright colors. For example, red means danger in many cases, and as a result of the conditioning that happened, our eyes go to red and other bright colors fast to find information about our environment.