Thank you for the kind feedback, everyone! Time to spoil the movie magic...
As most of you have guessed, the cloud effects were created by dropping things into a tank of water. I used water-soluble paint, and injected it into the water with plastic syringes. The scene was lit using 3-4 programmable LED lights, which I could change into any colour I wanted - typically one purple, one green and one blue. I found that as the tank got cloudier as I added more paint, I was able to completely change the background colour using light, allowing me to make colour compositions.
When the tank got too cloudy, I had to empty it out and fill it up again. You can see one of the syringes on the floor on the left, and obscured behind the camera are cups of various colours of paint. As you can see, I did this whole thing in the garage...
I have to give a shout out to SlothPaladin's cloud tank test videos, as they were my original inspiration for the look of the project. When I was first approached by Kaleb and he asked me to make a 4m35s stop-motion music video, I searched for shortcuts that would help me get the job done in time, and this was the perfect way to cut down the amount of actual stop-motion I had to do while still creating something visually striking.
The stop-motion portions were filmed under the same set-up, on the same desk but with the tank removed.
Regarding frame rate, I filmed most of the cloudy sequences at 50fps, which meant I could slow them down to 24fps without sacrificing any quality. I never experimented with dropping them to anything lower, as I didn't want to lose the graceful smoothness. The disparity between the live action and stop-motion frame rates is something I considered, but I figured that thanks to a decade of LEGO animation on YouTube, and now The LEGO Movie(s), most people are conditioned into thinking that LEGO people just move in this slightly jerky fashion, no matter what world they're in. It's part of the charm.
It felt good to be brickfilming again! The reason for my long silence is that my time and energy have been swallowed up by studying for my 3D Animation degree at university, which I will finally be graduating from next month. After that, who knows what comes next?