First, select your background layer (or make a new layer behind everything else if you don't have one). Fill this with one of your two colors. Make another layer just above it and fill it with the second. Now, you'll want to add a mask for this second layer, so go to Layer > Add Layer Mask > Reveal All.
This layer's look in the layers window should change. A second thumbnail will appear,
and a squiggly thing between them. The second thumbnail is a mask. What is white in
the mask is solid, and what is black becomes transparent. The squiggly thing is a link,
which means when you do things to one of the thumbnails, it may also happen to the
other. Turn this off, because we only want to do things to the mask. Clicking on either
thumbnail should select them. When the paintbrush icon is on the left, you're editing
the layer colors; when the icon is a circle, you're editing the mask. Be sure
the mask is selected for the rest of this effect.
Go to Filter > Noise > Add Noise, and turn its Amount slider all the way up. You
should get a speckled background between your two colors. We can smooth this out with
Filter > Blur > Gaussian Blur. The radius of this blur will affect the size of the
splotches.
When the blur is done, you'll notice that the two colors are very blended together. You can separate them again with Image > Adjust > Levels, and closing the arrows in around the obvious spike in the graph.
Now you can use various filters in the Displace and Blur menus to alter the appearance
of the splotches. In this demo, I used Radial Blur. And, once you're done, you can
always use gausian blur or levels to blur or sharpen your splotches.