Question: I need some code to convert standard C# TextBox to temperature TextBox which means adding "°C" to end of the text in the textbox with another color than the default color.

Answer: 
TextBox is a plain text editor. To get different colours you would have to muck around with a rich text box. Why not put the "°C" in a label positioned to the right of the text box? That would also make your parsing and rendering code much easier.
You could probably create your own control which inherits from TextBox and then override Text property to automaticaly add °C though other color inside the same TextBox could be problem. 

Why you want to have °C in TextBox ? 
Can't it just be label right after TextBox ? 
You can set static text and color to what you want.