Color is one way geologists classify soil.
Red soil can mean that there are iron oxides present, the case in Alabama and much of the South.
The Cereal Belt in the Tennessee Valley and the counties beyond have the richest soils which are the red clays and dark loams of the river valley.
In places that are hot and wet for much of the year, like Hawaii, red soil can indicate that water has removed much of the nutrients, a process called leaching, leaving behind very infertile soils.
I think red iron oxide came from a iron oxide dust cloud that passed thru our solarsystem in the last 10,000 years, take a look at mars, also the moon has a layer on the surface with the moon dust, on earthe the seawater played a role in piling it in cetain areas, but on mars were theres no rain it just stays on the serface.