White light contains light of all frequencies, so a violet paint that only reflected a single frequency would look very dark since it would only be reflecting a tiny proportion of the light that hit it. Even if it reflected a range of frequencies around violet it would still look very dark unless the range was large enough to also contain a lot of blue.

If you wanted to see "true" violet, your best bet would be to find a monochromatic violet light source. A Blu-ray laser would do the trick. But it won't look any different from the RGB imitation aside from being brighter.

EDIT: Please do not shine a Blu-ray laser directly into your eye.