For a while I've seen tutorials, especially military scale modelers, that use oils to add depth when weathering. Briefly the idea is that you put down a few small bits of the oils all over the model and then use turpentine and a flat brush to pull down the colors and blend them in to the model. In the end you get a nice color variation and a very natural looking background for further highlights like pigments or layers of dry brushing.
I've always wondered if I could get a similar result with gouache. Besides being great for lining tanks, gouache is water-based and dries quickly. Here is out guinea pig, a bit of scratch terrain from BoLSCon with a little spray paint red highlight and a sloppy assembly-line dry brushing.
Let's throw on some gouache, picking some reds/oranges and blues/purples to bring some depth to the piece. I guess I could have gone with some green as well, so don't be afraid to use some bright colors.
Now is the fun part, and where you remember that this hobby is more "art" than "science". Grab some water and start moving the gouache around. If you've let it dry then touch a wet brush to the colors to let them reactivate. I used a combination of tools for this step: wet flat brush (about 1/2" size), wet fingers, a small sponge and a damp paper towel. Some colors respond differently than others. My blues washed out quickly on the grey so I used the brush for them. The orange was very strong, so I rubbed it in with my thumb. In general you want to use a downward motion to give it a more natural appearance.
Now the best part of using gouache: grab your hairdryer and dry the piece off. It should take only a few minutes to get the colors on and blended and, depending on how much water you used, only a minute or two to dry off. If the effect is not quite right, add some more gouache and water. Because the piece will be dry when you are done, you can then move on to the next weathering technique.
Here's another side that received the same treatment. I used a little more orange on this side since I am going to rust it up later with some pigments. Since they are so diffuse, the gouache seems to stand up pretty well to handling, but you could always seal it with a coat of matte varnish for protection. I haven't worked with oils for weathering, but I have a feeling the gouache probably handles more like pigments and powders than oils, since the gouache is just pigment premixed with water.
Anyone find more creative uses for gouache out there?