Hi Csaba,
The model you stated and all other HP Laserjet colour printers use CMYK toners - CMY is a subtractive colour system ie: max CMY should give Black (the "K" or black toner is needed to provide true black and additional colour shades.
The white normally comes from the paper itself so to print white you would specify zero CMYK which with transparent paper will obviously not give you white
Hope this helps
Steve.B.