Zataar is a middle eastern spice. Its a kind of herb basically from the herb family of oregano, basil etc. Zataar is dried and made into a spice mix with sesame seeds, salt and some spices.
A zataar thats reddish brown in color is due to the sumac (dried red berry powder)
A friend on fooderati arabia educated me on this topic.
The green zataar is mild in taste where as the red zataar is a bit in sweet side according to myself.