Primary colours and secondary colours?

Question

I remember learning when I was young, a long long time ago, that the primary colours are red, yellow and blue, and this made sense for me all my life until now: red and yellow, give a range of orange, red and blue give a range of purples, yellow and blue give a range of greens, and there is the spectra. Now I found out that green is a primary colour, not a secondary one, and yellow is a secondary colour, not a primary one... Can anyone explain it to me why what I learnt as a child is no longer valid?