Something about removing pigment and exposing the original material. Apparently PLA is white with added dies.

There's other explanations on the web like "overheated air bubbles get trapped in the material thus changing the colour", but this one seems to be the most accepted. It's possible that they're both correct to some degree. I just wouldn't bet my life on the scientific reason just yet. As long as it works, that's all I care.

I always thought they used a two-layer material; black exterior over base white. A laser would remove the black layer exposing the white base. Something like what I bought to try out on the mini-CNC:

https://www.amazon.ca/Engraving-Doub...7QT93LN9/?th=1

I haven't tried them yet. They're a bit expensive, but the white base is semi-transparent; you can shine LEDs through them and create text with backlight.