The reality is you only need three of them and white. The three you need are magenta, cyan and yellow.
With these three you can mix any other one that you want. You need green, mix cyan, which is a more pure blue and yellow. You need purple, mix magenta, a more pure red and cyan. For black mix all three in equal amounts and there you have it. You can also mix all three in differing amounts to get brown.
We are told that red, blue and yellow are the primary colors and that these will give you all the others. But generally, red as we know it has a bit of yellow in it and when mixed with blue will not give you a pure purple. Also the blues that we usually see has either a bit of red or a touch of yellow in them and won't give you a pure mixed color either.
Artists usually get the already mixed hues because they think it's easier than mixing it themselves. But you will still have to add to that in order to blend with the artwork.
I use a limited palette of red blue and yellow because it's hard for me to find magenta and cyan in oils, which is what I use. However since really pure hues are hardly ever found in real life. I find it easy to mix what I need.
