In iOS 9,
U+2714 HEAVY CHECK MARK is included in Apple’s set of emoji characters. Just like the other emojis, it’s drawn as a full-color bitmap instead of a single-color vector glyph, so you can’t change its color with CSS. (In fact, there’s no guarantee that the check mark will even be black. Other platforms draw it in a variety of different colors!)
To get iOS to draw the check mark as regular text that you can recolor, you need to use a
U+FE0E VARIATION SELECTOR-15 character. If you put that variation selector character right after a ✔, iOS will use the regular text version (✔︎) instead of the emoji version (✔). OS X doesn’t have an emoji variant, so these look the same, but on iOS the variants look slightly different:
In HTML, you can add the character by putting a
︎ directly following your check marks.