Colors come in many varieties. Blue, red, green, yellow. We can use these colors in many ways, and associate them with various feelings, red with anger, or blue with sadness.
We can also mix and match them in unique ways to create designs. For example:
Monochromatic: Using various shades of the same color
Complementary: Using opposite colors on the color wheel
Triadic: Using 3 colors equidistant on the color wheel
Tetradic: Using 4 colors equidistant on the color wheel
These various combinations can really help in guiding a designer when building a website, or other forms of content.