i never really got into psp9 as you so not quite sure your referring to but i have a fair idea of what your trying to do though.
if your trying to correct the colours on a poster goto adjust tab/color/fade
The fade correct will restore your blacks to black and whites to white but you will need to mess about with the strength usage.
This is the tool i use for correcting dull white to bright white and the blacks also.
King kong before any work done to it as you can see it looks a tad dull
Now this image is exact same image except i used the fade tool correction was set to 80 here but you can clearly see the difference.
But you'd need to find the right correction strengths for different posters or artwork you are working it's abit of trial and error.