The font color used to generate the image. Can be RGB (255, 255, 255), Hex FF0000, or a number of CSS colors like red, green, blue, white, black, orange, and many more. See inc-flir.php for a complete list.
If FLIRStyle.inheritStyle is set the CSS font size will be used. This means you can use a style like <h1 style="color: red;"></h1> and have it work.