In design, color is one of the most important things to think about. It is in a matter of factly, one of the main reasons someone would pick one game, product, et cetera over another. Despite this, you can not go overboard and put random colors together. The most important things to know about using color is how to use it in harmony, how to provoke the right emotion, and how to make it stand out.
Color in general is one of the best ways to have something stand out. This is because, if used correctly, you can show a clear emotion and style in your project. An example is the use of the color red, which can be used to signify passion and excitement. It's used on Natsu Dragneel's Fairy Tail tatoo, along with orange, to show his passion for fighting, helping others and his friendly atmosphere towards them.
Once the main objective and emotion have been determined, you should know when to use a specific color scheme for that color. The analogous and monochrome —not to be confused with grayscale— color schemes are to be used when blending similar or the same colors, with similar objects to harmonize them. The complimentary and split complimentary color scheme should be used to provide contrast between two or more objects or subjects, with split complimentary being the more stable option to use and complimentary being very vibrant. This can also be used effectively for color properties such as warm vs cool, light vs dark, and saturated vs unsaturated colors.
Finally, when you determine how to use these color schemes and what colors to use, you can separate these into palettes. Palettes are groups of chosen colors used specifically to evoke a type of emotion or time set and can be changed to reflect the situation. This helps to maintain a consistent art style as you will always have the colors used for the project or previous projects. This is the most important way to define your design for a while.
Overall Summary
People sense different emotions from different colors
You can't just put random colors together
You should separate colors into palettes