If you already have a way to locate the characters then all you need are the TextFormat and TextField classes to achieve this. The TextFormat class allows you to define a style for the field...
var format:TextFormat = new TextFormat();
format.font = "Helvectica";
format.size = 14;
format.color = 0xFFCC00;
format.leading = 2;
Then you can set that formatting on a subset of text in a TextField using...
var field:TextField = new TextField();
field.text = "fish cat dog rat"
field.setTextFormat( format, 6, 12 )
... where 6 is the index at which to begin formatting and 12 is the index to end it.
If you don't have a way to determine the start and end points of the formatting I'd suggest looking into using regular expressions.