Why can't change fill color if text setting [None] color by SDK?

I have TextFrame setting color =None

Now, i want change color to swatch name ="TestColor"

But my code can't apply,

My code below:

ICharInspector oCharInspector = textRange.GetCharInspector();
ICharFeatures oCharFeature = textRange.GetUniqueCharFeatures();

AISwatchRef swatchRef = sAISwatchList->GetSwatchByName(NULL, ai::UnicodeString(strColorName.c_str()));
result = sAISwatchList->GetAIColor(swatchRef, &fontColor);


sAIATEPaint->CreateATEApplicationPaint(&fontColor, &fillColRef);
IApplicationPaint oAppPaint(fillColRef);
oCharFeature.SetFillColor(oAppPaint);

//Apply 
textRange.SetLocalCharFeatures(oCharFeature);

If i change color of Text to other color (Ex: Black), My code working ok.

Why can't change color if Text setting none color?