how to use string.fontcolor() ?

Hi,

I want to put a value of a button dynamically with a fontcolor.
The value is some text and a variable part which is the selected value in a
Select.

<INPUT id=go TYPE="submit" value="">
....
xval=new String()
xval=document.getElementById("mySelect").value //selected value =460
document.getElementById("go").value="this value " + xval.fontcolor("green")
+ " is in green"
....

I get: this value <FONT COLOR="green">460</FONT>is in green
How to get the value really in green?

Thanks
bob