richtextbox

Even if you're scripting in the font using the appropriate RTF codes, the RTB won't recognize it as a valid RTF file unless it begins with a group containing \rtf. In other words, the first 5 bytes should be {\rtf

If it's not working properly this might be the reason?

I'm afraid that's not possible.

A plain text file can only store the characters of the text, not how they look.

If you need to store font, color etc. then you can create an .rtf file using Wordpad or Word and the RichTextBox can load it directly as this its native format. Of course, you can give such a file a .txt extension if you like though, personally, I wouldn't do this as it's likely to lead to confusion in the future.

I need to save text with font and color in .txt file and open in richtextbox with same font and color. 

I'm not completely clear what the problem is but to load an .rtf file into a RichTextBox, I'd use:

   richTextBox1.LoadFile(filePath);

To load a plain text file instead, I'd use:

   richTextBox1.LoadFile(filePath, RichTextBoxStreamType.PlainText);