In at the deep end with a rather technical first post. Sorry folks!
I've been trying to track down the origin of the fairly widespread contention that cool brown dwarfs are magenta in colour. It seems to come from: Burrows et al. The theory of brown dwarfs and extrasolar giant planets. Reviews of Modern Physics 2001; 73: 719-65. (text is here). In section VII D, they say:
They don't give a reference for which program they used or how they used it.Indeed, the recent measurement of the spectrum of the L5 dwarf 2MASSW J1507 from 0.4 to 1.0 [microns] (Fig. 21 ...) indicates that this L dwarf is magenta in (optical) colour. This is easily shown with a program that generates the RGB equivalent of a given optical spectrum (in this instance, R:G:B::1.0:0.3:0.42, depending on the video "gamma").
But a look at the relevant Fig. 21 spectrum suggests that something is wrong with their derivation (examine it here).
The optical bit of the spectrum is between 4000 and 7000 Angstrom. Notice the flux on the vertical axis of Fig. 21 is logarithmic.
For the human eye, blue receptor sensitivity is maximum at 4400A; green at 5350A and the red receptor peaks at around 6000A. So we're getting blue stimulation at a normalized log flux of vaguely -2.2 (in the midst of that fuzz of spectral lines); green at about -2; and the red receptor sees very little at its peak sensitivity, but gets the advantage of the flux rising towards -1 at the long wavelengths towards 7000A. Converting the logarithms, red wavelengths are peaking at 10 times the green and blue peak energies.
Tightening up on that arm-waving argument a little, I sampled the flux graph at sixteen points across the visual range, and summed the X,Y,Z chromaticity vectors for those sixteen wavelengths, weighted by their relative flux, to find a final approximate colour coordinate for the spectrum. That final XYZ value converts to RGB at around 1:0.1:0.1, depending on your monitor white point. (The underlying MathCAD sums are tested and true, since they accurately reproduce the well-known colours of black-body spectra.) In agreement with the handwaving argument above, that final colour is very distinctly red.
So: I think there's a flaw in the "magenta" story, but I may be missing something. In particular, it would be nice to know if anyone is aware of the spectrum => colour program Burrows et al. used but failed to reference.