Change text color by scripting

Hi all,

I am using this code to change the color of a text layer. 

var mySourceText = layer.property("Source Text");
var textDocument = mySourceText.value;
  
textDocument.fillColor = RGBcolor;
textDocument.strokeColor = RGBcolor;
  
mySourceText.setValue(textDocument);

It works fine if the text has the same style. 

If not uniform, it changes the text style to match the first character style.
For example, if I want to change this text to green:

This is what I'd like to get:

However, this is what I get:

Any ideas?