Messenger For Web
Open Messenger and select a chat conversation from the right. There are three new options above the Mute Notifications option that let you set a nick name for the person you’re talking to, change the chat color, and change the emoji.
Read: Hide Your Active Status On Facebook Messenger
The chat color changed on your end will change for the other recipient too but it’s on a per-conversation basis so it won’t change for other conversations. The emoji change works the same way.
Messenger App
Navigate to a chat conversation and tap the contact name at the very top. Three new options have been added below the ‘Notifications’ switch that let you set a nickname for the other person, change the chat color, and pick a different emoji to replace the default thumbs up button that appears in the app.
Another curious thing we noticed is that if you tap and hold the emoji, it gets bigger. The longer you hold it down, the bigger it gets. When you release the button, an enlarged emoji will be sent. The change, both in color and emoji, will happen on both sides of the conversation so it isn’t just you changing a preference for yourself.
If you don’t see the color and emoji customization options in the app, make sure you’re running the latest version.
Change Chat Colors & The Emoji Shortcut On Facebook Messenger