Corners, How do I sew in Gimp?

[Krikor]

Another way:
Let's say you want the frame to have 8 different colored borders.
Then you create 8 empty layers, and from top to bottom you fill each color as follows:
With the top layer selected Select - All;
then, Select - Border, and enter the border value and finish with Edit - Fill With FG Color.

Having done the previous step, select the bottom layer and repeat the process, just double the border value.

In the third layer, the same thing, just triple the border value; quadruple it in the fourth layer and so on with each subsequent layer.

   

It is possible to speed up the process even more by keeping the 'Change Foreground Color' window open and choosing the color and dragging it to the created border selection, instead of using Edit - Fill With FG Color.


PS: Don't forget to merge the layers at the end of the process to form a single layer with all the edges