When text in the Atto editor is formatted with any HTML tag, the accessibility checker tool consistently reports a color contrast problem, regardless of whether or not color formating has been applied to text.  In other words, basic black text on the white background of the editor, when it is surrounded by <p> tags or has bold or italics applied, is reported as a color contrast problem.  As there is no way to remedy that situation, the error is confusing to end users.