The soil near the surface is usually darker than about one meter down, it is because the top soil

The soil near the surface is usually darker than about one meter down, it is because the top soil is drier than sub-soil.
Subsoil is the layer of soil under the topsoil on the surface of the ground. Like topsoil, it is composed of a variable mixture of small particles such as sand, silt and clay, but with a much lower percentage of organic matter and humus, and it has a small number of rocks that are smaller mixed with it.
Topsoil is drier as it is in direct contact with the air and sunlight which makes it drier and darker.