con_color is a client command and you shouldn't change the client con_color without is permission. The way you want to put color in a chat message is to:
1) #include <colorchat>
2) download numb colorchat.inc and put it in scripting>include
3) use this format:
PHP Code:
ColorChat(index, COLOR, Msg, any...);
Example:
PHP Code:
ColorChat(id, RED, "This Is Red,^x01 This is Normal,^x04 This is Green");
^x01 = user con_color, default:yellow
^x03 = color mentioned, RED in this example.
^x04 = green color