Sugar is usually made either out of sugar canes or sugar beets. Cane sugar might not be vegan as some companies use char derived from cow bones as a filter to whiten sugar during the refining process. Beet sugar, however, is refined in a different way and bone char filtering is never used in its processing.
Finding out which brand of Sugarcane is vegan is not an easy task. Usually there is no information about the refining process written on the package, and knowing the brands does not help either as several manufacturers might be providing sugar produced with different processes for the same brand. Using brown sugar is not a solution as brown sugar is often refined and made brown again.