Before electronic markets, the National Quotation Bureau printed inter-dealer OTC quotes on pink-coloured paper, giving rise to the name 'pink sheets'. The market still exists under OTC Markets Group as the Pink tier and is split into Pink Current Information (some current disclosure), Pink Limited Information (limited or stale disclosure) and Pink No Information (effectively dark).
Pink-tier names include legitimate foreign companies that choose not to dual-list, distressed US issuers, shell companies and the long tail of micro-cap speculative names. The combination of light disclosure, low share prices and thin volume makes Pink-tier stocks the highest-risk segment of the public US equity market — and the segment most often associated with pump-and-dump activity.
Most mainstream brokers either block the Pink Limited tier entirely or impose restrictions: cash-only trades, no online order entry, broker-assisted only, or pricing surcharges. Day-trader-focused brokers like TradeStation, Cobra and Webull provide broader access at the Pink Current tier.