The complete C45 reference guide is here.
I have a C88 using the same driver setup linked above and there is no adjustment for omitting background colors.
That is normally adjustable through the software using the printer. For example, you can tell IE to not print background color through the advanced menu section.
The only thing that comes to mind is that you possibly have ICM (Image Color Matching) selected in your Epson color management settings. That would tell the printer to match the colors as closely as possible to exactly what you see on your screen. It is not the default setting however. I haven't used Irfanview in years (use photoshop) but are you using a white background?