You can use the option ViewPoint
ChromaticityPlot3D[
"RGB", "RGB", Appearance -> "VisibleSpectrum",
PlotLabel -> Column[{"RGB x RGB", "Rotated 180\[Degree] around vertical"}, Center, 0],
AspectRatio -> 1, ImageSize -> {256, Automatic},
ViewPoint -> {1, -1.5, 2}
]
For ChromaticityPlot3D the default setting is {1, 1.5, -2}, with x (Red) as vertical.
