All the colors of light mixed together create white. The three primary colors the eyes pick up are blue, red and green. When all the colors of light (red, orange, yellow, green, blue, indigo and violet) are mixed together, the human eye sees them as the color white.
Red, blue and green (the main stimuli for human color perception) are considered the primary, additive colors. Additive colors are created by mixing together a variation of spectral lights. Specifically, additive colors are only optically mixed when spectral lights are placed closely together, they don't truly mix. This differs from the process of subtractive color perception, which is formed when pigments absorb specific wavelengths of white light, while reflecting others.
Secondary colors are seen when two of the primary colors are mixed but one is excluded. For example, red mixed with green creates yellow, green and blue creates cyan, and blue and red make magenta.
When the primary additive colors are combined at their most intense states, the result of all the colors of the electromagnetic spectrum mixing is white light. While the RGB color model is flawed at encapsulating every color we perceive, it does explain the perception of white light.