Mixing oil paints is not a precise science. Ask six painters to make the colour turquoise for you, and you will end up with six similar, but fundamentally different, shades. To mix the shade of turquoise that you are looking for, it is helpful to use a guide. Keep the picture or colour swatch next to you as you paint. The basic components of turquoise are green and blue. Once you mix your base, lighten or darken the hue until it matches your example.
Mix equal parts blue and green paint to make a basic turquoise.
Lighten the turquoise, if necessary, by adding white paint. Add small amounts at a time and mix well. Do your best to keep track of the amount you add, so you can repeat the process later.
Give dimension to the colour or match it accurately to your example. Add small amounts of blue, grey, green (or any other colours) to achieve the specific shade of turquoise you are looking for.