import fl.text.TLFTextField;
import flashx.textLayout.formats.TextLayoutFormat;
import flashx.textLayout.elements.TextFlow;
import flashx.textLayout.elements.ParagraphElement;
import flashx.textLayout.elements.SpanElement;

var my_fmt:TextLayoutFormat = new TextLayoutFormat();
var myTextFlow:TextFlow = my_txt.textFlow;
var paragraph:ParagraphElement = myTextFlow.getChildAt(0) as ParagraphElement;
var span:SpanElement = paragraph.getChildAt(0) as SpanElement;
my_fmt.color = 0xFF0000;
// myTextFlow.hostFormat = my_fmt;
span.format = my_fmt;
myTextFlow.flowComposer.updateAllControllers();

my_txt.text = "あいうえお";