One of the first lessons you learn in school is to put a comma between your adjectives, just like I'm doing when I say I'm truly, madly, deeply in love with commas. I'm lying, but at least I'm grammatically correct. But here's the problem: that lesson is wrong. You don't always put a comma between your adjectives. You only put them between coordinate adjectives. With cumulitive adjectives, you don't. Yes, I'm about to explain what that means.
Little Brown Ball
You should always put commas between coordinate adjectives, which are descriptive words that are similar to each other. For example, if the painting is black, brown, blue and green, those adjectives coordinate because they're all colors. They all go together and fit in the same group. There is a simple way to test if adjectives are coordinating or not.
Add and. Is the painting still the same if it is black and brown and blue and green instead? To further test your adjectives, switch them around and see if the sentence still works. Can the painting be blue and black and green and brown? If it can, you've got some coordinating adjectives on your hands.
If you do not have coordinate adjectives, you're working with cumulative adjectives. This is where the little brown ball comes in. The ball can be little and it can be brown, but those two adjectives aren't the same. One is describing the color of the ball, the other the size. They don't coordinate. Therefore, they don't get split up with a comma.