That doesn't add colour to, say, the smiley face, it just changes the colour from the default, usually yellow.

As well as adding colour browsers display emoji at a larger point size. It is really annoying when some pre-emoji symbol (like the phases of the moon in TFA) gets emojified and where your document had some discreet use of symbols it now has huge colourful icons drawing the eye.

And afaict there is no way of turning off this behaviour globally.