(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}]

![RGB components of visible spectrum](https://i.sstatic.net/RNdnb.png)

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"`.

---

**Addendum:**

Just to make this post less useless, here's a *Mathematica* implementation of [Bruton's conversion algorithm](http://www.physics.sfasu.edu/astro/color/spectra.html):

    brutonIntensity = Interpolation[{{380, 3/10}, {420, 1}, {700, 1}, {780, 3/10}},
                                    InterpolationOrder -> 1];
    
    brutonLambda[x_, γ_: 4/5] := Map[N[brutonIntensity[x] #]^γ &, 
        Blend[{{0, Magenta}, {3/20, Blue}, {11/40, Cyan}, {13/40, Green}, {1/2, Yellow},
               {53/80, Red}, {1, Red}}, Rescale[x, {380, 780}]]] /;
        380 <= x <= 780 && 0 < γ <= 1

Here's a gradient plot:

![gradient plot for Bruton's scheme](https://i.sstatic.net/Wqa0Z.png)

and an RGB component plot:

![RGB components for Bruton's scheme](https://i.sstatic.net/1DxiU.png)

I haven't fully investigated the method for converting wavelengths to CIE xyz coordinates; maybe one of these days...