The example linked is mostly a simulation through cloud effects, likely with some extra amorphous particles added for a more natural handmade paper look.
Generally when I need to make more colorful clouds, I will fill an initial plate with a normal cloud then continue filling it with inversion clouds (using those colors, or others depending on how crazy you want it.) After several passes the edges will become fairly hard so you may need to smooth things out and desaturate the color a bit. Applying a circular gradient wash over the piece will help create the halo effect in the example. As for the particulate, clouds can be used to generate them too, but it takes a bit more fine tuned work and trickery.
If you can find a higher resolution example you'd like to copy, I can probably come up with the technique.
Although I couldn't find a high res then that, I could just buy it.
But before I do something like that I need to know one other thing. How could I make images tillable? What's the trick to doing that? Do you just make the top and bottom rows of pixels identical? How do people usually do something like this?
The requirements needed to get a graphic to tile nicely varies depending on the image composition building it. Composites simply need the elements to wrap correctly, nothing special really. In the end, that's what all tiling graphics require. Singular photographs are the most difficult to get to tile well often. Patterns are often somewhat easy, but composites are very simple.
If I was to make something like that I would look around for cloud-like brushes. I would then make a full layer of this pattern at about 50% opacity in black or any colour. Then Id get the same brush/similar brush, harden it a bit and use it as a rubber to bring out the edge of the canvas. You can use filters to get it the orangey/yellowy colour from black/white.
Just play around. You'll be able to get something similar pretty quickly. I certainly wouldn't ever pay for a digital image.