As paper ages it starts to get browner as its fibres decompose. The speed of degradation depends on the quality of the paper in the first place but this process is speeded up by sunlight, acidic conditions created by poor quality mount board or wood and by changes in temperature and humidity. The sailing ship at anchor had a mount which protected the edges, probably from direct sunlight. The other picture of boats (below) has vertical lines showing the edges of the 3 bits of wooden backing behind it.

Once the paper has started to go yellow the colours appear changed. Reducing the discolouration can help to restore the original colours and balance of a picture as can be seen from the portrait of the man.