Red soil colour is caused by :

Red soil, often referred to as the 'omnibus group', derives its characteristic reddish hue primarily from the presence of iron compounds, specifically ferric oxides [1]. These soils develop on crystalline igneous rocks, such as Archaean granite and gneiss, in areas of low rainfall [3]. The red color is a result of the diffusion of iron in crystalline and metamorphic rocks through the process of oxidation, where minerals combine with oxygen to form oxides like hematite (Fe2O3) and limonite [4]. While the top layer appears red due to these iron oxides, the soil may look yellow when it occurs in a hydrated form, such as goethite [3]. In contrast, when these minerals are in an environment lacking oxygen (reduction), the red color can turn greenish or bluish-grey [4]. Iron and manganese are the primary elements responsible for imparting these distinct red and orange colors to the soil [1].

Sources

1. [1] Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 6: Soils > 2. Red Soils > p. 10
2. [2] Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 6: Soils > 9. Grey and Brown Soils > p. 12
3. [3] NCERT. (2022). Contemporary India II: Textbook in Geography for Class X (Revised ed.). NCERT. > Chapter 1: The Rise of Nationalism in Europe > Red and Yellow Soils > p. 11
4. [4] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 6: Geomorphic Movements > Oxidation and Reduction > p. 91