In my opinion, the eye loves a pattern. And loves to see it interrupted, changed, or broken.
I'm betting most of you like the blue one best. It's not that you like blue. It's because that blank has the most consistent pattern.
Most casting videos show people pouring resin into a flat tray in a zigzag of successive colors. Even if you get good color separation (and many don't), it's all random. No discernible pattern on the finished pen.
I'm trying to pour in a way that has some intention, hoping to create the patterns that we find pleasing. I pour all the colors simultaneously, and very slowly. I only pour 6 blanks at a time, because of how long pouring takes. I'm working on a solution to pour all 6 simultaneously at the optimal time and temperature.
