Banana leaf paintings aren't really made from the banana leaves. It is actually made from strips of the bark of a banana tree, but a banana tree is not like most trees we know. Their trunk is actually layered similar to an onion. Villagers peel strips off the banana tree and there are many variation of colors of the strips which depends on the age and type of banana tree. An artist would let the banana strips dry which changes the colors. Then they cure it to keep the color permanent. They would painstakingly use the different colored strips in their painting. Making a banana leaf painting is a time consuming process and the artists are devoted to their crafts. See many examples of banana leaf paintings here.