You can also do this by binding attribute of Textbox take a look
TextBox1.Attributes.Add("onmouseover", "javascript:this.style.color='gold'");
TextBox1.Attributes.Add("onmouseout","javascript:this.style.color='black'");
[Edited]
hmm! It's my mistake that I was taking question as change color of text of textbox on mouseover
You should follow the answer of
Toniyo ,
Dalek as well as
MKgoud if you will have to change text of control then my Idea will work.
In short you need to use
onmouseover="this.style.color='gold'"
instead of onmouseover="this.bgColor='gold';"
[/Edited]