What does Synthetic Dyes mean?
A human-made organic dye.
Heddels explains Synthetic Dyes
The first synthetic dye, mauveine, was discovered by accident by William Perkin, who was actually trying to create quinine (a drug used to control fevers) from coal tar.
Since then, they have replaced natural dyes to a large extent, being cheaper and offering more vibrant color choices. Synthetic dyes are divided into subcategories such as acid dyes, azoic dyes, pigment dyes, and reactive dyes.
In terms of raw denim, synthetic indigo is often used to replace natural indigo.
Yellow cloth being dyed with synthetic indigo: