Batch watermark and/or layer adding?

Hi!!

Although I've been GIMP for a while now, I've only now started learning how to use it properly.

My issue is that I want to add a colored watermark on a directory of images. I've stumbled upon BIMP and also on the many Batch watermark plugins found here, but I've had several issues.:

1) BIMP won't let me select a directory or read JPG files (the default camera extension)
2) The batch watermark plugin (Batch Tools Watermark) won't let me define a color on the watermark (even after attempting modifying the script). In addition, I also can't replace the Batch Tools Watermark file. As in, I've found several here but GIMP only allows me to use this one (Batch Tools Watermark), even if I replace the file for another within the same plugin folder. Am I doing something wrong?

Is there a way of saving a layer and applying it to multiple images? Or could I change the watermark color by changing/adding something to the watermark scripts found here?

Thanks for your help!!!
Marta