What does Overdye mean?
A dyeing process most frequently used on indigo or black denim fabric.
Heddels explains Overdye
Overdyeing is a process where the fabric is either dyed for too long or dyed a second time. It is most often used on denim to add an overtone of color to the indigo.
Elhaus Warbonnet 16oz denim, overdyed with sulfur: