(too long for a comment)
Plot[{ColorData["VisibleSpectrum"][x][[1]],
ColorData["VisibleSpectrum"][x][[2]],
ColorData["VisibleSpectrum"][x][[3]]}, {x, 380, 750}, PlotStyle -> {Red, Green, Blue}]
It doesn't seem that you'll be able to obtain Yellow (RGBColor[1, 1, 0]) from ColorData["VisibleSpectrum"]; unfortunately, the docs say nothing about how they're blending the colors to produce "VisibleSpectrum".