What was the name of the British colony that is now Ontario?

The British colony that is now Ontario was called Upper Canada.

Upper Canada was created by the Constitutional Act of 1791, which divided the former Province of Quebec into Upper Canada and Lower Canada. Its territory covered most of present-day southern Ontario, along with parts of northern Ontario.

The word “Upper” referred to geography, not political rank: Upper Canada lay upstream along the Great Lakes and upper St. Lawrence watershed. Lower Canada, centered on the lower St. Lawrence, roughly corresponds to modern Quebec.

Upper Canada’s capital moved from Newark, now Niagara-on-the-Lake, to York in 1796. York was renamed Toronto in 1834. In 1841, Upper and Lower Canada were united as the Province of Canada, so Upper Canada stopped existing as a separate colony.