I don't do anything special to the epoxy. As the photos show I just add a single drop of paint before I mix it. I have no idea if it affects the strength, I have no way to test it. But I haven't had any problems. I have noticed that it increases cure time.I've never tried coloring the epoxy. Have you seen any modifications that you needed to make to the balance of ingredients to allow for the addition of the acrylic paint? Does it effect strength or curing time? I do like the effect the black tube and painting inside of the blank has on the final pen - it seems to add more depth to it. Nice work!