GuiProgresssCtrl does not use a bitmap. You can change it's border and color and stuff, by making a new profile for the ProgressCtrl and setting it to that profile.
Like if you made this:
new GuiControlProfile(MyProgressProfile : GuiProgressProfile)
{
fillColor = "255 0 0 255";
};
and then you set the ProgressCtrl's profile to MyProgressProfile, then the color of the bar would be a solid red.