A word from Old English
Fallow is a pale, yellowish brown, like dry grass or sandy soil. The word goes back to Old English fealu, which meant pale yellow or light brown. German still has the related words fahl and falb. Maerz and Paul date its first use as a color name in English to the year 1000.
The fallow deer is named after this color. Its coat is a pale reddish brown with white spots in summer.
Not the same word as a fallow field
A fallow field is plowed land left without a crop for a season. That word comes from a different Old English word, fealh, and the two only look alike. Some people say the color is the shade of a dry, unplanted field. The Wikipedia article on fallow calls this a story made up after the fact.
One code, several names
#C19A6B is the ISCC-NBS light yellowish brown. This site gives the very same code to camel, wood brown, desert and lion. Those names are much younger. Wood brown was first recorded in 1886, camel in 1916 and desert in 1920.

