That doesn't really make sense as asked. A String is just text. It doesn't contain any information about how that text should be displayed unless it is interpreted as markup, e.g. RTF or HTML. You can instruct a Label how to display the text as a whole, e.g. by specifying a font or colour, but variable formatting takes something more.
The Label does have a Content property but then you'd need to assign something to that that could do the variable formatting. There's certainly no standard control that will allow you to load plain text and easily specify how it gets formatted. The RichTextBox does it by loading RTF markup and interpreting that. Another option that is still not the simple magic solution you want but might be easier than RTF is to use some sort of web browser control and use HTML markup to format. That's a bit more human-readable than RTF and web browsers are read-only by default.