As you are using a paint event and its graphics object, it works just like doing it in any other graphics object (like the one a canvas has).
So you need much more code and a bit of effort, eg determine the width of each part that you want to have, set the colour for the first one, draw it with DrawString, set the colour for the second, draw it at the position where it needs to be, etc
You could impress everyone by coming up with a general method that takes a StyledText and draws it