screen shots, please.
Pure white in RGB mode is 255 in each channel. Converted to print (CMYK) it becomes 0 -- no ink, not white ink.
I suggest that you print a very light gray to simulate white... something along the lines of C3 Y2 M2 K0 which translates to approximately equal 148 of R, G and B. Depending upon your printer and ink and paper surface, that number may change.
One thing more, in the event that you are considering offset lithography, the lithographer, on occasion, is asked to print white ink on colored paper stock. In that case, since a single litho ink film is only 1 to 2 microns in thickness, it usually takes two or more overprints of white to hide the paper color. Moreover, when a simulated white panel is requested on white coated stock, it is not unusual to print the panel in gloss or dull varnish so that the difference in shine creates the panel.