I think this is an optical phenomenon and no chemical reaction is taking place. When you are looking at a solution, you are basically perceiving its absorption spectrum. When you have a solid, you are looking at the reflectance spectrum and the story is more complex, if you have a heterogeneous solid like this stuff.
If you originally had the solution and you froze at home, this is slow freezing and the size of ice crystals could be larger (phase separation). If you had a chance to look at frozen Coke/Pepsi, it does not look like "liquid" Coke or Pepsi. It is much lighter, light brown rather. Ice crystals are to be blamed for this color change. If you froze Pedialyte very fast probably, the ice crystals would be smaller and the color change would not be that drastic.
The second effect could be the solubility of this particular dye at freezing temperature. If you had an optical microscope you would probably see the segregration of particles. Since you are saying that other "colors" do not show this effect, most likely, this pigment is not very soluble at low temperatures.
Also note that certain anthocyanins (common flower/fruit colors) change color when frozen because at a low temperatures, a different conformation of the dye molecule exists, but this is rare. This is an extreme example of a dye changing its color drastically upon freezing. See for example, Unusual Color Change of Vinylpyranoanthocyanin−Phenolic Pigments, J. Agric. Food Chem. 2010, 58, 7, 4292–4297.Here
