Any detection of electromagnetic radiation is actually tied to its frequency as this is what atoms respond to when interacting with radiation. The colours of the spectrum one sees are therefore an indication of the frequency not the wavelength. The latter could only be determined by interference e.g. if one uses a diffraction grating to disperse the light, and I can hardly imagine that a line redshifted from the blue into the red part of the spectrum should in fact show a dispersion like a blue line.
www.physicsmyths.org.uk
www.plasmaphysics.org.uk
