I have about 30 different colors - but I don't actual NEED all of them. I just see new colors and have to have them!!! I think Wilton has a color wheel that shows how to make different shades from some basic colors. So I guess it depends on whether you want to mix your own or buy them already mixed. Good luck!
But when I buy some I'm going to start out with the primary colors for simple designs and buy the specific colors when the need arises. HTH. 
I don't have any airbrush colors because I don't have an airbrush but as far as the icing colors go for dying the buttercream I have TONS!!!! I bought a few of each when certain ones were discontinued like Aster Mauve and Cornflower Blue and another one I can't remember the name of it. I have about 4 Dora Flesh tone lol because I needed alot but it turned out it didn't work for what I needed it for. I have one no taste red but plan to get a few more of those incase the store ever stops carrying it or something lol. I have the top of my tool box full of them, plus my extras are in the bottom of my tool box plus I have a box of Americolors that is in the bottom of the tool box. I needed SUPER black so I purchased the Americolors online and then decided to use chocolate icing instead of black lmao.... But it's good to have them and if they are the Wilton paste colors you can use the glycerin stuff to revive them if they've gotten hardish from sitting too long 
I admit, I have about 40 bajillion (rough estimate) Wilton colors that I use in buttercream and fondant. I don't know why I think I don't need 40 bajillion airbrush colors. Maybe it's because the Wiltons were a gift, and I have to actually buy the airbrush colors!
I have about 15....I have all the primaries....some Neon..flesh...browns....and Kopykake teal that I already had.I just bought all new Americolors before Christmas.
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