If it's a 2D panel.. locate the panel background bitmap in the panel folder.. open with an editor (like PaintShopPro), and edit away ..

If it's a VC.. it's a little trickier. The theory is the same (edit the bitmap), but it's not always clearly named. It would most likely be an extended bitmap with a "C" in the name (filename_C.xxx)

The extension will tell you what type of file it is (extended bitmap or DDS). Those require something like DxtBmp, or ImageTool, to open.

Be aware there might be an alpha channel.. and if it's a DDS file, there is the "flipping"  (if you open with DxtBmp, and save with ImageTool, you'll have to flip the bitmap vertically (top to bottom)).

Edit..  I see that I'm in the FS9 forum..   Nevermind about the DDS stuff.. but you might still need DxtBmp to open, and then save the edited bitmap..