Sorry, the answer to your question is that your button is neither Confederate nor Union. It is what button-collectors call a "flower button" or "Fashion button," made for use on fancy Civilian-usage coats. Yours is known as a brass 1-piece button, and its backmark saying "Warranted Rich Orange" in RAISED letters means it was manufactured in Britain sometime from about 1790 into the 1830s. (The term "warranted" in Brit-speak means "guaranteed"... and the term "Rich Orange" referred to the color of the gold gilt on the button.)
Although these 1790s-1830s brass 1-piece "flower" buttons were still around during the civil war, they were never a Military-issued button. We relic-diggers do find some in civil war campsites, because a "needy" soldier would sometimes replace a lost Military-issue uniform button with whatever type of button was available.