Why do some hearts render in colour and others don't?

Because they are different kinds of character. The outlined and solid hearts are text symbols and take the colour of the text around them, so they turn white on a dark theme. The coloured ones are emoji, which render as fixed images and stay red whatever the theme.

Which heart is safest to use?

The solid black heart and the white outlined heart. Both come from Miscellaneous Symbols, both predate emoji by years, and both are covered by essentially every font shipped today.

Heart symbols, copy and paste

Tap any symbol to copy it. 27 characters, each one verified against the Unicode database.

Ready-made combos

Tap to copy the whole arrangement. These are the framed forms people use in bios, built from characters that are checked the same way the grid above is.

Dividers and borders

Box-drawing characters join without gaps when repeated, which is what makes these read as one rule rather than a row of dashes. Each character counts against your limit, so a long divider is an expensive piece of decoration.

In a bio

Three lines you can adapt. Replace the words and keep the spacing.

Hearts are scattered across several Unicode blocks rather than gathered in one, which is why they vary so much in weight. The outlined and solid forms from Miscellaneous Symbols are the oldest and most widely supported. They render as ordinary text characters and take the colour of the text around them. The coloured ones are emoji: separate characters that render as full-colour images, and which cannot be recoloured.

Which heart to actually use

Three are worth knowing by name. The black heart at U+2665 and the white heart at U+2661 both come from Miscellaneous Symbols, the same block that gave us the card suits, and both have been in Unicode since 1993. Font coverage for them is effectively universal. The heavy black heart at U+2764 is younger and is the one most systems now draw as a red emoji rather than as text.

That last point catches people out. The same shape can be a text character that follows your theme colour, or an emoji locked to red, depending on which codepoint you picked. On a dark theme the text hearts turn white and the emoji stays red, which is why a bio can look considered on one phone and mismatched on another.

What they cost

Text hearts count as one character. Emoji hearts often count as two, because they sit in a part of Unicode that some platforms measure differently. On a 150-character Instagram bio, five emoji hearts can quietly cost ten.

Where they fail

The floral and rotated hearts at the start of this set are decorative punctuation from older printing traditions. They are handsome and poorly supported, so treat them as a nice-to-have rather than something to build a profile around.