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?

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.

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.