Here's my first GIMP Tutorial: How to easily make a plaid background with matching colours.
With your image open go to
Search Depth can be lowered ... you don't need so many shades of the colours for this. You'll get a little stripe of colours. Copy it to the clipboard.
Create a new image. Fill with the created pattern from the clipboard. (It's usually the top
left item in the pattern dialogue). Fill your image with it. Duplicate that layer.
Layer>Transformand rotate 90° ( no matter what direction).
Now it depends a bit on your liking. You can set the opacity of the rotated layer to 50% or
change layer mode to overlay or whatever you like. Then merge down. Crop to the plaid
area and that's it. You can then make it all seamless to use it for larger areas
This way you ensure the matching colours.