force_hex_style_colors

This option enables you to control TinyMCE to force the color format to use hexadecimal instead of rgb strings. It converts for example "color: rgb(255, 255, 0)" to "#FFFF00". This option is set to true by default since otherwise IE and Firefox would differ in this behavior.

tinymce.init({
        ...
        force_hex_style_colors : false
});