I will stand by you can't get a paper white area on a color darkroom print, if the negative is of normal density and the exposure is of a reasonable time--it will tone up slightly.
I guess we must agree to disagree then.
But if that is what you want, then by all means, lower the maximum RGB value to 250 or so.
Other than putting a black (or any other colour) edge on the image, say 5 pixels or even less, I don't know of a way.
I don't subscribe to reducing the already limited dynamic range of the paper to something even less by artificially cutting back the image data.
I remember a fashion for very lightly fogging paper before it was placed under the enlarger and I never saw the sense in that either.
You could expose the entire paper briefly, so that it was just under the threshold. That way any amount of additional exposure would print. Helped with very dense negatives.
A similar technique could also be used to "lift" shadow details in a slide: Do a double exposure of a neutral, even subject (graycard) at "-2 and a bit" (experiment). That will just lift the shadows without effecting the lighter tones (much)