"To the red country and part of the gray country of Oklahoma"

The western part of Oklahoma has a very red soil due to its high iron content.  Northwestern Oklahoma is nicknamed ‘Red Carpet Country’ because of the soil, and the Red River that runs between Oklahoma and Texas takes its name from the red sediment that colours its waters. 

In eastern parts of Oklahoma the soil has a more normal brown and gray hue.  The Verdigris River that runs through Northeast Oklahoma takes its name from the green gray sediment.