Because we are used to panchromatic black & white these days, not everyone realises that old photos are often of a different kind of black & white. E.g. a tram on an old photo looking dark, could have been yellow.
The colours of visible light are determined by the wavelength. The spectrum goes from shorter to longer waves in the order violet – blue – green – yellow – red. With old photos often orthochromatic emulsions were used that are little sensitive to light of the longer wave lengths like red and yellow, but very sensitive to short wave lengths like blue and violet. Because of this red and yellow look very dark on many old photos while blue and violet turn very light. Later often yellow or red filters were used to compensate for the difference in sensitivity for the colours and finally panchromatic emulsions became common, that are more equal sensitive for all colours.