Dull grey, like a mule's coat, sturdy and unremarkable.
Mule is a neutral grey.
It can be described as neutral, medium, colorless and almost grey.
Set in black, because black reads best on it.
The mule is a domestic equine hybrid between a donkey and a horse. It is the offspring of a male donkey and a female horse. The horse and the donkey are different species, with different numbers of chromosomes; of the two possible first-generation hybrids between them, the mule is easier to obtain and more common than the hinny, which is the offspring of a male horse and a female donkey.
Read on WikipediaA mule or courier is someone who personally smuggles contraband across a border for a smuggling organization. The organizers employ mules to reduce the risk of getting caught themselves. Methods of smuggling include hiding the goods in vehicles or carried items, attaching them to one's body, or using the body as a container.
Read on WikipediaProper noun