, the API calcium test kit sucks. You can probably shake it between drops but 1 way or another the reagent needs to be mixed. The Salifert says to gently swirl.
The end point should be blue. If it's purple you added too many drops and went past the end point.
You may want to look into the electronic Hanna calcium checker. It costs around $50 but it gives you the exact reading on a digital display. The is no guess work as to when you hit the end point color.
Hanna Checker - Test Kits - PRODUCTS - English
I actually live about 100 miles north of Philly near Wilkes-Barre, PA.