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).
style "panel"
{
fg[NORMAL] = "#FFFFFF"
engine "murrine" {
textstyle = 0
}
}
widget_class "*Panel*" style "panel"
widget "*Panel*" style "panel"
class "*Panel*" style "panel"