Save this code as `~/.gtkrc-2.0` and restart your XFCE panel with `xfce4-panel -r`

This file will set your panel text color to white (overriding the color setting from your current GTK theme). Change the `#FFFFFF` to another color if you want another color.

The `engine "murrine"` part will remove the text style for Murrine based themes (for example, a drop-shadow effect used in the Ubuntu Ambiance theme and Greybird).

```gtkrc
style "panel"
{
    fg[NORMAL] = "#FFFFFF"

    engine "murrine" {
        textstyle = 0
    }
}

widget_class "*Panel*" style "panel"
widget "*Panel*" style "panel"
class "*Panel*" style "panel"
```