The ice palette is a fascinating idea. I wonder if the cold retards the paint drying or the condensation of humidity?
I think that RealGenius' idea is correct. I had already reached a similar soultion to keeping paints wet on hot summer days using two plastic jar lids of different sizes. The smaller one was turned upside down and had crushed ice in it, and over this was placed an upright larger lid with a flat surface for mixing paint. Less evaporation occured because there was less heat energy to change the state of the water in the paint. But interestingly, water also condensed onto the upper lid, showing that the evaporation reaction could work in reverse, where the heat energy in the gaseous water in the air was lost in heating the ice, and it changed into liquid water in the process, making the paint even more liquid at times.On a side note, I have found that the temperature of the model itself affects the wet blending of colours, where warm models dry quicker with perhaps less blending, and cooler models have more time for the pigments to intermingle. Just a thought.