Colorimetric Test for Trace Evidence. This simple, two part presumptive test will help you quickly determine whether or not a surface has recently been exposed to a discharged firearm. Simply wipe down the surface in question, add the chemical reagents, and observe the results. A rapid color-change takes place to verify the presence of nitrates, the main component of gunpowder residue (GSR), while a different color change will verify the presence of heavy metals associated with GSR. Each kit contains complete instructions.