Possibilities: The solvent (70% ethanol) can extract or denature some pigments. This is selective and affects some pigments more than others. The original color was a blend of several pigments and purple is the color that was left.
I had the same observation, but it was only after 1 day or less in ethanol.
Collembola were'nt the target of my study, so I didn't paied a lot of attention to that color. I only noticed they were purple and thought it was their natural color. If that can help.