Here's a 10 paint palette for oils aimed at painting whatever you want, it's just colors:
Titanium White PW6
Cadmium Red (orange biased red)
Quinacridone Magenta, Quinacridone Rose or Alizarin Crimson (purple biased red and true primary)
Cadmium Yellow (orange biased yellow)
Yellow Ochre (must have)
Burnt Sienna (awesome warm earth tone)
Ultramarine Blue (your purple biased blue)
Azo Yellown Hansa Yellow or Lemon Yellow (your green biased yellow)
Cerulean Blue (green biased blue)
Raw Umber (great cold earth tone)
To that you could add some comfort tubes, like Lamp Black, Burnt Umber, Raw Sienna, Phthalo Blue, etc.
With the 10 paints above you can paint pretty much everything.
cheers