Can anybody share the experience with the antitryptic activity assay using botanical extracts? Which standard and in which concentrations should I use to obtain the standard curve?

In my method, I am mixing trypsin with the extracts, afterwards adding casein as a substrate for trypsin. After incubation time I am stopping the reaction with 70% perchloric acid, which results in a color change - depending on the plant species, I am observing blue or pink coloration. For standard drug - diclofenac sodium, I am not observing any coloration. Is this difference in color a normal phenomenon? Please advice.

Jadwiga