如何在“富文本”框中更改新添加文本的颜色

Consider my windows application built using C# VS 2005 displays certain contents in a Rich Text Box. Now when i add some new text, it should appear in a different colour. i.e i need to differentiate the text which gets displayed programmatically and the text that i enter in the text box. how can i do this.?Consider my windows application built using C#