The measurements below were made using a caliper, which was squeezed together tightly, then released to see how the felt jumps back.
Thin green cloth
This is a simple woven cotton material, which is a single layer. The change in sound is quite subtle, with almost no muting effect. The hammers hit an extra layer of dampening, so the result is a mellow, characteristic timbre.
Black satin cloth
On this piece of cloth, one side is satiny, the other is like an elastic band. Due to the thicker nature of this fabric, the hammers give a more thumpy sound, that separates from the sustain of the strings. The result is a soft mallet, vibraphone-like sound world.
Baby blue felt
Well, in life it has a tiny blue hue. This cloth has a thin felt front and a black backing that isn’t as cushioned as felt. To the touch, it has a vegan leather feel. This backing reacts to the vibrating string, giving it an extra zing. Hence the interesting clang-clang sound, like an electric piano.
Thick white felt
This is just an “ordinary” felt, also commonly used inside the piano. It is uniform on both sides, with little hairs sticking out – these make it very soft, giving a lot of cushion to the hammers. This results in much more dampening, therefore a big volume drop, and is the homage to the middle pedal of the common upright piano.