All the colors we see are combinations of red, green, and blue light.
Visible light may be a tiny part of the electromagnetic spectrum, but there are still many variations of wavelengths. We see these variations as colors. On one end of the spectrum is red light, with the longest wavelength. Blue or violet light has the shortest wavelength. White light is a combination of all colors in the color spectrum. It has all the colors of the rainbow. Combining primary colors of light like red, blue, and green creates secondary colors: yellow, cyan, and magenta. All other colors can be broken down into different combinations of the three primary colors.(If you're mixing material like paint, the primary colors are red, blue, and yellow... but for light, the primary colors are red, blue and green!)
Objects appear one color or another because of how they reflect and absorb certain colors of light. For example, a red wagon looks red because it reflects red light and absorbs blue and green light. A yellow banana reflects red and green light, and absorbs the rest.