The yellow in sepia-tone photographs was explained to me some years ago, when I was puzzled by an image of Colour-Sergeant Anthony Booth, V.C. of the 80th (Staffordshire Volunteers) Regiment. For an example please see http://www.rorkesdriftvc.com/intombi.htm
This would be because the early photographic negative emulsions (collodion) were sensitive only to blue light - other colours rendered oddly (to our eyes). Yellow and any warm colours would show up as dark, for example, while blues and whites will be very light. This is one of the reasons that people often think ladies are in mourning in old photos, whereas in fact they may have been wearing bright orange dresses.
From the mid 1880s onwards, with the development of gelatin dry plates, dyes were introduced to the emulsion that extended the sensitivity to other areas of the spectrum.
