Most likely what you create with moldable plastic will need color. Polly Plastics has color pellets (sold separately) to apply to your plain plastic which will help you create a vivid consistent color. However, it isn't the only method to apply color to the plastic. I provide coloring options that I have used and details below.
The full options include color pellets, paint, color alcohol ink, or paint powder. Note: an option to avoid is using food coloring, it won't give you the results you want.
Using Color Pellets
A helpful color guide comes with your order and it will tell you exactly the number of each color pellet to get the desired hue. The pellets are dropped in the hot water with the meltable plastic pellets and then worked into or kneaded together. I find that it is easier to flatten the plastic and use your fingers to break the color pellets, then knead it, rather than clumping it into a ball right away and kneading to blend the colors. Also good to know, the plastic pellets are white in color when they dry, but you will get a better, brighter white when you apply the white color pellets. The photo above contrasts the color pellet example on the left and the one without color pellets on the right.
You can also use acrylic paint, spray paint, or a Sharpie type marker once the plastic is hardened. You'll want to finish with a polyurethane to prevent flaking and chipping for the long term. I show here painted plastic on the left compared to using the color pellet on the right.
Either method will successfully provide the color you need for your project. If you aren't as skilled at painting, the color pellets are a good option to get the desired effect.