Our eyes have three types of color-sensitive cells, or cones, each specialized to one color: red, green, and blue. These colors lie in order on the visible light spectrum. Despite their specialization, the cones generally combine their forces and activate in the presence of more than one color. Green and orange, for example, both activate the red and green cones, but in different ratios.
Related: What Color is The Sun?
Of course, not all colors are in the light spectrum as we know it: brown, for instance, is not a spectral color, but a combination of many different colors on the spectrum. When you see brown, you're seeing a mixture of light wavelengths that activate different cones in varying ratios to produce a color your brain finally interprets as brown.
Related: What Makes Fireworks So Colorful?