https://www.fool.com/investing/2021/09/23/sec-cracks-down-on-pink-sheets-used-by-wolf-of-wal/
The Pink Market — named for the color of paper it once published share prices on — is where investors trade stocks over the counter rather than on a major exchange like the NYSE. Most of these are so-called "penny stocks," selling for less than five bucks per share. Some really do only cost a cent.
While certain retail investors love the rollercoaster experience, trading in pink sheets is considered highly speculative (because many of the companies are in bad financial shape) and dangerous (because, as the SEC has warned, the off-exchange market is rife with fraud and manipulation).