If the instructions fit on a two pages [1] and people can do it at home with no training, I think they should be able to to cover it in a day certification for professionals.
That's the instructions set for one product. Now enter a barber shop and start counting.
You also need to know all the interactions between products, and between products and skin, and between product and skin in the context of the goal of the sitting client, which might want a color point in between two tints with reflex of another.
You also need to know all the interactions between products, and between products and skin, and between product and skin in the context of the goal of the sitting client, which might want a color point in between two tints with reflex of another.