As has been said already: this and other colour systems (bayer) have an amount of bleed.
Foveon color-bleeding is much worse than Bayer.
Bayer sensors use optical color filters, which are much, much better than silicon in filtering â€˜undesiredâ€™ wavelength bands.
In a Bayer sensor thereâ€™s color bleeding because of 'cross-talk'.
Cross-talk is when light passing through the color filter of a pixel excites electrons in neighboring pixels.
This kind of bleeding is perfectly correctable and there are different techniques that do it.
In contrast, the color bleeding in a Foveon sensor is uncorrectable.
Like I said, silicon is much worse at filtering light of certain wavelengths compared to an optical filter.
(It's completely another matter that manufacturers use wider bands for the color filters in order to improve sensitivity - which, of course, results in poor color separation.)
You gotta admit: the wiki link you posted earlier on during our little skirmish, perfectly matched what was said in the white papers - R & G & B; nothing said about white, yellow and red.
Osiris is actually right about this one.
What you see on the Foveon web site is a logical diagram of how light is filtered.
The diagram certainly does not represent how the filtering is performed in practice.
As Osiris said, all of the layers in a Foveon sensor are equally sensitive to all wavelengths.
But based on the depth of a layer, only light of a certain wavelength band is supposed to be absorbed by this particular layer.
In practice, though, the absorption is far less than ideal, so color separation is (much) worse compared to using an optical filter.