Accidentally mixing a small amount of paint in to a 5 gallon pail will not affect its color or sheen. Typically a quart or less will have little or no affect on the pails color, but if a pure white paint is mixed with a very dark paint the color can be lightened.

Make sure to mix the paint well then test its color and sheen by applying some paint to a piece of card board and allowing it to dry. You can also apply some paint from the 5 gallon pail to an area that has already been painted. Choose an area that is easy to repaint in case there is any problems with the color.