How Do you mix foundations to make the right shade for a...

[Ritamichele]

If i can't find a shade for a client, how do i alter/mix shades i already have to help match a clients skin tone??? Do i Need other products in order to change the shade of a foundation, well i did buy Coastal Scents Liquid Color Corrector in black,white,green,yellow,and i think red, and read only a couple reviews people had on the item where they used the BLACK to darken a Foundation, White to Lighten but i dont know how to mix liquid color correctors and foundations, i need help