Here is how it is done. Feel free to try and publish werever you like:
I had not written the scirpt for that, but it is an easy one, just
calling the described filters and procedures in the order described:
1) Paint horizontal strips of the desired colors on a blank image.
I fashioned a specially crafted gradient file for this, but it could
be done in a lot of ways: painting by hand, repeated use of
filters->render->pattern->grid (zeroing the width for the vertical
strips) . Another way: Sinus fiter, getting horizontal "sinusoidal"
patterns, followed by map to gradient, followed by
filters->colors->map->to gradient, followed by index image using few
colors, or desired palette (and them back to RGB image), and a couple
of other ways.
2) Create a new layer and fill it with the Sinus filter
(filters->render->pattern->sinus. Zero out the "y" range (second
slider) for vertical strips.
3) If desired, warp lightly the sinus layer with Iwarp
4) Filters->map->displace, on main colored layer, using the sinus
filled layer as parameter. Turn off X. Increase Y displace to about
160 - 200px.
5) If desired, apply iwarp fitler on resulting layer.
6) For making it tileable and increasing the "noise", use