The material comes from the rivers. Rivers come out of the mountains carrying lots of material. As river water meets ocean that material is deposited. It drops out of the water. The heaviest material, sand, drops out first.
Lighter material drops out later.
We also have a current running from the bottom of the image to the top right corner. We're focusing on how the coastline is constantly being changed. Lighter, aqua blue, is likely to be shallow. We think they are areas of mud which have dried out and as the H2O evaporated left behind salt. Salt makes the white colors.