Adding Coloured Border to Image

[rich2005]

All sorts of ways. This on the traditional side.

Choose a foreground colour
Image -> Canvas Size
Set the Width and Height equal.
Click on 'Center'
Set resize layers to All layers
Set Fill with to Foreground Color
Click on Resize.

example: https://i.imgur.com/fEvYPXE.mp4

If you have lots to process, well there are Batch utilities or the Command Line ImageMagick utility.