As explained in the link provided by Otaku here , current text editing tools cannot handle this (unless you go for the hacked OP in this Q, which seemed like not a good solution).
You can write code to do it manually yourself, completely edit the text editing control, but that would probably mean a lot of work. It took Microsoft years to get support for combining diacritics that work properly in MSWord. I would look for open source software that has this feature, and see how other developers did it. This may be difficult to find, and you may have to go beyond .NET-land. Maybe OpenOffice can do this?
This discussion may also help.
I'm afraid that you will find that you have to manually parse Unicode and assign colors to the correct glyphs. If you want to be complete, this is one feature of the job.