I did something like this over 53 years ago, except the paint was poured into a sink filled with water & then you laid the paper on top. A family friend was getting her elementary teaching degree & she wanted my brother & I to do some craft projects she might use in class. I was maybe 8 years old then. All I remember is she had a sink full of water, poured the paints in & we laid thick paper on top. Then you lifted it out & laid it to dry. They looked a lot like the photos you posted. I'd guess they no longer do this.