Yes, the highly decomposed organic matter soil is darker.
Explanation:
The color of the soil is obtained due to the presence of minerals and organic matter in the soil. The highly decomposed soil looks dark because the organic matter turns to black when it reaches to the final stage that is called humus. Humus (decomposed organic matter) is dark in color due to the presence of the sodium in the organic matter. Sodium readily covers over the soil and it makes soil to look darker in color. Thus the "highly decomposed organic matter" in the soil looks darker.