This is a great question. The standard has always been that the capacitive buttons on Windows Phones, Lumia or otherwise, are intended to be white. For whatever reason, that has not been the case with 928 buttons. They have varied from yellow - white. This is the first I'm hearing of a "glowstick green" color.
The theory is that the capacitive buttons are yellowish because the adhesive didn't/hasn't tried correctly. One poster said it may be helpful to take a blow dryer to the phone for a bit while making sure that the battery doesn't get too hot. People have reported that the color gets better over time. Nokia hasn't yet publicly accepted that the difference in color is a defect.