The British spelling of the word color as colour in the backmark means your flatbutton is definitely British-made. Brass 1-piece flabuttons with indented-lettering backmarks first start showing up around 1810... but the War-of-1812 interrupted imports from Britain, and the war made British goods unpopular in the US until around 1820.
Brass 1-piece flatbuttons mostlly got superceded in public popularity by the advent of machine-made inexpensive "ornate" stamped-brass 2-piece buttons around the mid-1830s.
So, it's "most likely" that your British-made flatbutton was manufactured and imported into the US sometime in the 1820s to very-early 1830s.