It's mixing time. Be careful when working with the pigment not to breathe any of the very fine powder (wear a mask or respirator), and work on a surface that can be cleaned, such as a wax paper covered workbench.
It only takes a very small pinch of powder per test tube. You can mix the pigment and water in each test tube -- use a stirring rod or bamboo skewer to thoroughly mix it, or place a stopper over the top, cover the hole with your thumb and shake it like crazy.
If you plan to use the same color in a number of test tubes, premix a batch in a measuring cup, mason jar, or beaker and then pour 50mL per test tube.
Shine a UV LED into your tube to see the effect!
The pigment is suspended in the water, so you may need to occassionally shake it up to redistribute, or mix in a tiny amount of an emulsifier such as xanthan gum (available in health food stores and online) to keep it in suspension.