You need to explain what process you are carrying out. I presume you're not using auto white balance? A simple and fairly foolproof way is to get a white or 18% grey card and take a shot of it. I'm not familiar with the E-300 though and not sure if it has an option to take a WB reading in such a way. Here are a couple of links that may help:
CUSTOM WHITE BALANCE--E300 OLYMPUS
EVOLT E-300 Using White Balance Indoors