system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When i use red or blue shower gel/shampoo, why is the foam always white?
gpt
Foam is made up of small bubbles, the walls of which are very thin.  The vast majority of the light either reflects off the front surface, or reflects off a surface one or two bubbles deep, so the total distance through the soap is minimal, so the dye has little chance to absorb light.