Why does the red heart emoji require two code points, but the other colored hearts require one?

It appears that the red heart emoji (❤️) "\u2764\uFE0F" requires two Unicode codepoints, specifically Heavy Black Heart followed by a Variation Selector. However, blue 💙, green 💚, yellow 💛, and purple 💜 each have their own single codepoint.

Why is red so different?

Solution 1:

For historical reasons. Originally, there was only U+2764 HEAVY BLACK HEART which the first applications that supported Emojis decided to render as a red heart. These early applications always rendered U+2764 as Emoji. Later it was realized that this was a bad idea and the variation selectors for Emojis were standardized. When additional heart emojis were added, there was no need for another red heart, so it was omitted. Instead there's a separate black heart emoji U+1F5A4 🖤.

In theory, an application could require that the Emoji variation selector is appended to other heart code points as well. But it doesn't make much sense to render characters like PURPLE HEART as a non-Emoji. It does make a difference for HEAVY BLACK HEART, though, which is often intended to be rendered as the original, plain heavy black heart character.

Solution 2:

HEAVY BLACK HEART was added to Unicode decades before emoji. When emoji were incorporated in Unicode 6 some already existing characters were simply reused as emoji to avoid unnecessary duplicates. Later, variation sequences were defined for characters that also map to a non-emoji character set to allow for better control over how they display. For example, U+2744 ❄ SNOWFLAKE is originally from Zapf Dingbats (I believe) but was later also made an emoji. So if you want to force the original text-style display you can use VARIATION SELECTOR-15 (resulting in ❄︎), and if you want to force the newer emoji-style display you can use VARIATION SELECTOR-16 (resulting in ❄️).

Note, however, that not many platforms actually support those variation sequences correctly at the moment. Also not all of them automatically apply the variation selectors when using the emoji keyboard. In theory ❤ and ❄ (and many other emoji) should display as text style by default without VS16, but many applications ignore that as well.

I have a list of all code points that can display differently via a variation sequence, on my website, if you're interested. The next Unicode update in June is going to add some more.