You COULD produce "white" using just a red laser, a green one,
and a blue one (i.e. all VERY narrowband) so you've effectively
got just 3 wavelengths present! BUT the colour rendition would
be AWFUL if you used it as room illumination, AND you could come
up with coloured cards, or filters, that looked (say) green in
sunlight, and looked TOTALLY BLACK under this laser "white" light -
for obvious reasons.


It won't look totally black, colored cards or filters are not nearly narrow
enough for that, but the color rendering will not be perfect if the emission
bands don't exactly match the sensors in the eye. You can in fact have very
good color rendition with three fairly narrow emitters, but the problem is
finding a good red phosphor. The modern Yttrium based phosphor used in color
CRTs and trichromatic fluoescent lamps is orange-red, rather than a pure
deep red. It's much more efficient and longer lasting than the old true red
phosphor but the CRI will not exceed the mid 80s using it.

Generally speaking, a light source that does a good job rendering reds will
have good all around color rendition, red is the hard part.