The first thing to do is gather a few bottles or transparent jars and decide what colors we want to paint. It can be a single color gradient that goes to white, or gradient between various colors.
The next step is to mix colors to achieve the desired tone . I used watercolors , but you can also use wall paint, if you prefer. Dilute the paint with a little water to better flow into the bottle (but not too well or it won't cover), and absorb with a large syringe ( you can buy it at any pharmacy ) .
Then, pour the whole picture in the bottle with the aid of syringe . Large bottles need between 80 and 100 ml. paint to cover comfortably ( then the surplus is not discarded) . Move the bottle for the paint to flow and go covering every corner .
Once covered , let the bottles upside down several hours draining any excess paint can be within a sink or on absorbent paper.
After that, we give them back, clean the excess paint from the edge and let dry completely ( to me it took me a couple of days ) .