Do you have the means to test for coagulase in your lab? S. aureus will be coagulase (+), S. epidermidis and others will be coagulase (-). You may have access to TGA (Tellurite-Glycine Agar) plates, which differentiate coagulase + (black pigmented colonies) and coagulase – (grayish, or white)although it’s certainly not perfect. By "whitish" in pigment, I’m assuming you have already tested on TGA, or do you mean regular nutrient agar? The latter usually doesn’t tell you much.
