Green, orange, and violet. The secondary colors are those three colors that are mixtures of the 2 adjacent primaries. When newton discovered the components of the spectrum he announced there were 7 colors. The number 7 was considered to be magical at the time and so he ‘saw’ a 7th color to fit the number. When color theorists started making color wheels the indigo didn’t make sense, but with the number reduced to 6 it fell into place. The spectrum is a product of the way we see. We have 3 color receptors in our eyes and the 3 primaries are their equivalent in paint. The brain then recognises the 3 intermediary hues as distinct basic colors. We see these as separate colors in the spectrum, and all color mixing is based in the interaction between these 6 colors and the tones and shades of them.