1950 Census -- how was information on "Race" gathered?

I ask this because I'm curious about a change in response. In the 1930 and then 1940 census, my grandparents are marked as "white" even as they give their origin as the British West Indies. However in the 1950 census, they (and their children) are marked as "negro". Would this mean that they themselves gave that different answer, or would the census taker have made that decision/mark that down on their own?