To achieve your desired shade of color, you should always start by mixing a small amount of the necessary colors on your palette. It is important to not take a large amount directly, since there is no guarantee that you will create the shade that you want on your first try itself.
You should also use water to dilute the watercolors as and when required. Even after you feel satisfied with the shade that you have created on your palette, it is best to test it out on a piece of white paper first to ensure that it looks like the shade that you were aiming for.