The design is distinctly Art Nouveau, and the "Colonial" mark has been noted on other Art Nouveau dress buckles of the early 1900's:
http://www.worthpoint.com/worthopedia/art-nouveau-buckle-circa-1900
There are also early 20th century Egyptian Revival dress buckles bearing the "Colonial" mark.
Although a number of firms of that period may have used "Colonial" as a proprietary mark, one of the likelier candidates for this buckle could be Smith & Crosby of Attleboro, Massachusetts. Established in 1874 by William H. Smith and Alfred H. Crosby, the company specialized in jewelry and fashion accessories: cuff buttons, fobs, knives, chains, etc. They remained active until at least the 1930's, and "Colonial" is definitely known to have been one of their brand names.