M

Census takers from 1850 to 1880 filled a color column for each person. In 1850 they left it blank for white people and wrote B for black and M for mulatto; by 1870 they also wrote W for white, C for Chinese and I for Indian. The 1870 and 1880 instructions defined mulatto to include anyone with "any perceptible trace of African blood."

The entry was the enumerator's judgment, and the same person can be marked B in one census and M in the next. Treat it as a clue about how a person was seen, not as a fact about ancestry.