What causes the different colours of the salt pits in the Saloum Delta in Senegal
Writing about the San Francisco Bay salt ponds, Dr R. J. Rusay notes:


  Due to variable algal concentrations, vivid colors, from pale green to bright red are created. The color indicates the salinity of the ponds.  Micro-organisms change their hues as the salinity of the pond increases.  In low to mid-salinity ponds, green algae are predominant. In middle to high salinity ponds, an algae called Dunaliella salina shifts the color to red. Millions of tiny brine shrimp create an orange cast in mid-salinity ponds. Other bacteria such as Stichococcus also contribute tints.


Rusay says nothing about the Saloum Delta, but the explanation could be similar there.