For my outline shader, I started off with a plain thick line. That looked a bit amateurish. I moved to a thinner line that used the scene's shadow color, which meant purple during the day, red at sunrise and sunset, and dark blue at night. I was never totally satisfied with how that looked. Now I think I have something I like a little more.
I take the pixel color, normalize it (so if you think of the RGB channels as a 3D vector, make that vector length 1, so you have the same color, but total brightness is 1), multiply the original pixel color by the normalized color, and you end up with a saturated version of the original color. Now multiply by the shadow color, and you don't just have a purple line over everything, but you have a darker, more saturated color of the pixel, that's pushed toward the color of the scene's shadows the same way everything else is: by multiplying the color.
And here's a first look at the new desert-themed assets:
These models and most of the textures started life at the Unity Asset Store. I put them into Blender, redid a bunch of bad UVs, made custom collision meshes, vertex painted them, then made custom materials in the engine. In addition to my usual shader that changes color with time of day, I added wind direction, and the fabric and branches move in waves with the wind, going up and down and stretching slightly in the XY plane in the direction of wind. The red vertex color, multiplied by wind strength, tells the mesh how far to move. The blue vertex color tells the mesh if it has to stop moving downward at a certain point, so the fabric meshes don't drop below the wooden frames. It looks really good in close-up, like it's a physics simulation and not just shader instructions.
In World Machine I also made some desert mesas that you can see the foot of in the background, similar to how I made my temperate climate mountains.
I think I need to make the scene more colorful overall. I should take a look at other games that have good desert scenes and see what they did to make everything not the same color.
I also replaced all of my old combat particle effects with the new stuff that I learned from DucVu FX. For fun I made a level that plays a bunch of the new effects at random: