In September 2015 I already wrote a post containing a piece about artificial aging of concrete, stone and the like with a moss recipe. Apparently it doesn't work all that well. The moss dries out quickly, and the photo that I linked to was rigged: someone had cut out some moss and pasted it on a wall, something quite different from growing moss.
Hence, this message about how to make it succeed with lichens. Lichen is a symbiosis of a fungus and an alga, or a fungus and a bacteria (blue-green algae). Today I came across this link in which the following recipe is given for culturing lichens:
Recipe for lichen:
½ litre of milk, 1 teaspoon of flour, 1 teaspoon of yeast, ½ teaspoon of gelatin, and 1 tablespoon of green algae powder (or 1 tablespoon of dried mushroom powder), and a tiny pinch of water-soluble fertilizer.
Bring almost to a boil, and let it cool until it is lukewarm. Then add:
2 teaspoons of lichen flakes, collected from common lichens growing in a similar microenvironment to where the new lichens are wanted, plus 2 teaspoons of healthy soil from the area near where lichens are currently growing. Stir well.
Use a paintbrush to apply to the surface where you want the lichens. Additionally, lichen covered rocks can be placed on top of it, in hope that they will shed some propagules. Last ingredient is patience!
So… if you somewhere still own an ugly wall and have a lot of patience, then this may be the answer! I wonder if you can make wall paintings with lichen. There are the craziest kinds and colors, but those will of course over time overlap each other causing such a painting to fade. You can even eat some kinds of lichen! On the forum however is cautioned against eating all the yellow and yellow-orange lichen, because those can be toxic.