Oil pigments are suitable for coloring the encaustic medium. You will need very little to get saturated colors. You can use the pigments just as they come out of the container, you do not need to grind them or anything of the like.
In addition … for (hot) encaustic
To color the  encaustic medium, in addition to using pigments for fatty media, you have to consider that they are stable at working temperatures and low toxicity.
Buy pigments
To buy pigments for the encaustic painting you must go to specialized stores of Fine Arts. Fine art materials follow strict controls of light resistance. Good brands publish lists of suitability of their products for the different techniques. There are some very widespread pigments in oil paint that should not be used for hot encaustic painting like Prussian blue, as it decomposes at temperatures around 70 ° C.

You should look for pigments that  are pure and are not mixed with fillers. The encaustic painting is a very transparent medium and the fillers muddy the result.

Except for primary colors and white, we advise you to buy the smaller size container, encaustic is the paint that less colorant requires for the same color intensity .