Some notes about Tru-Color Paint:
Tru-Color Thinner works for both Airbrush-ready and Paintbrush-ready formulas. The Airbrush-ready paint can be brush painted, but it is thin and will require multiple layers to cover well.
Because the bottles are plastic, some solvent may have escaped through evaporation. Fill the bottle with thinner to just below the neck and that should be the correct amount of solvent to pigment. Of course experiment to see what works best for you.