💀 or ☠️: which one you meant
Pick 💀 for tone and ☠️ for content. The plain skull has drifted almost entirely into humour, so a 💀 after a story reads as a reaction to it, not a threat. The crossbones version still carries its old printed meaning of poison, lethal danger and piracy, which is why it shows up on posts about toxic plants, spicy food challenges, chemistry homework and true crime. Swapping them changes the message more than people expect. "This hot sauce ☠️" warns your friends off. "This hot sauce 💀" says it already defeated you.
Why ☠ sometimes loses its colour
☠ was designed as a text symbol, so its default presentation is plain text. Emoji keyboards normally attach an invisible character, variation selector 16 (U+FE0F), that tells the receiving app to draw the colour version. Copy it from an old document, a font picker or a symbol chart and the selector is often missing, and you get a small monochrome skull that matches the surrounding letters. 💀 has no text form at all, so it never has this problem. ☢ radioactive and ☣ biohazard behave exactly like ☠. If a pasted crossbones looks flat, pick it again from your phone's emoji keyboard.
From poison bottles to Mr. Yuk
The skull and crossbones marked poison bottles long before computers, and it is still the GHS pictogram for acute toxicity on chemical labels. It also had a known weakness. In 1971 the Children's Hospital of Pittsburgh introduced Mr. Yuk, a scowling green face, and the usual explanation is that local children linked the skull to pirates and adventure rather than danger. The emoji inherited both readings. ☠️ on a post about cleaning products means keep away, while the same glyph beside 🏴☠️ and 🦜 means a costume party.