When the rendering engine is HTML, for every tokens of a color schema you can also set a custom inline CSS style. Some basic CSS style can be handled also by the RTF engine, but for a best view you must choose the HTML render engine. For this reason the preview of the Color Schema always uses the HTML engine.
You must insert a CSS style valid for the HTML style attribute.
Please note that the custom style is not put inside the HTML style attribute but embedded on the global <style> tag inside the class style definition of the token. So you can define a custom CSS style sheet that override the inline settings.