The other, more interesting, answer is that I use a little paper folder (mine is pictured above) that I made by heavily coloring in squares on some heavy watercolor paper with water-soluble crayons-- Caran d'Ache is the brand I have, but any water-soluble crayon or pencil will do. I use a water-brush (also in the picture) filled with clean water to wet the color I want to use, and then I paint in the sketch. To clean the brush between colors, I squirt a few drops of water out of it into my hand. I carry the folder in a pocket in the cover of my sketchbook/journal, but it could also just be slipped into a page anywhere in the book. Sometimes I remember to slip a piece of waxed paper into the folder in case any of the paint squares are wet when I close it, but mostly I don't do that. The water-brush is as easy to carry as a pen. I can hold the book and brush and lay the folder opened out on the opposite page. It's really quick and easy to do.
I've used the folder shown above since early May when I went to Portugal and France with this journal, and it's just now needing some refilling of color. To refill, I just add more color to the squares. You can make some squares bigger than others or even make several squares of your most-used colors. You can also mix these paints just like any other. I often add a little white to my colors as I'm painting to make them show up better on the darker paper in my journal.
On the right at the top is another not-really-damp drawing, my friend's cat Montana sampling some of the prosciutto we were enjoying with melons and peaches. But down below you can see the damp factor: in the large pot of my beleaguered Meyer lemon tree (which is still holding on to one of its lemon babies and also has sprouted a new flower) is a tiny mushroom farm. The mushrooms are bright yellow, like the lemons are supposed to be eventually. They smell so strongly of damp that I could smell them through the screened door that I was sitting inside of.
I wandered out to the front porch in search of more damp residue and found a paper bag that was droopy and limp. It hadn't been rained on, but it had absorbed so much humidity that it didn't have its crisp paper bag qualities at all. I flopped it on the kitchen counter and drew it. And then I went back outside and investigated the ice plant by the front porch, where my husband had seen the strange phenomenon of slugs apparently trying to get to higher ground. Sure enough, a whole troop of slugs were clinging to the tops of stalks of the ice plant. They hadn't eaten any leaves, and they don't usually go on the ice plant. But their favorite lettuces and strawberries were in flooded beds, and the sturdy ice plant stalks were still standing up more or less straight. As much as I don't like slugs, I left these pitiful guys to their own devices and just drew them.