Why FDE colors are different？

Because there are no standard specifications for FDE color matching, you'll find it called different things. GLOCK calls its finish Coyote Brown. You'll also find it called Desert Tan and Coyote Tan with lighter and darker variants. You can see and discuss the differences on this AR15.com thread.

Because there are no standard specifications for FDE color matching, you'll find it called different things. GLOCK calls its finish Coyote Brown. You'll also find it called Desert Tan and Coyote Tan with lighter and darker variants. You can see and discuss the differences on this AR15.com thread.

As shown in the figure below, they all claim to be FDE colors, but they look very different.

“FDE” is used throughout our catalog to reference a spectrum of colors that include tan, brown, bronze, earth, and everything in between. Due to the chemical process that takes place during Type III Hardcoat, the final finish can vary from part to part (even parts done in the same batch). Therefore, your “FDE” product may vary from other parts from SPECPRECISION.COM and other companies, and may vary from the photos shown online.

If you receive FDE with different colors, please rest assured that it is not a quality problem. It's normal.