You can customize your conversations on Facebook Messenger by selecting a unique color and replacing the Like button with a different emoji. These changes take effect immediately and apply to everyone in the conversation. The color change will not be visible on Facebook, but the emoji will be displayed.
Steps

Open the Messenger conversation where you want to change the color. You can alter the color of any conversation in Messenger. The new color will apply to the conversation title and message bubble. Everyone in the conversation will see these changes.
- The color change is only visible in Messenger; you won’t see the color change when viewing messages on the Facebook website.

Open the conversation details. The action differs slightly between iOS and Android:
- iOS - Tap on the name or the participant list at the top of the screen.
- Android - Tap on the ⓘ icon at the top-right corner.

Tap on "Color". This will open up a selection of color options for your conversation.

Select the color you want to apply. The change will take effect immediately, and the conversation title and message bubbles will switch to the new color.
- Participants in the conversation will be notified that you’ve changed the color, and they’ll see a "Change" link allowing them to quickly select a different color.

Tap on "Emoji" in the conversation settings to change the emoji. The default emoji is the Like (Thumbs Up) button next to the message input field. You can replace the Like button with a new emoji in the conversation. This emoji change will apply to all participants in the conversation.
- Swipe left and right to browse available emojis. Once you select a new emoji, a notification will appear in the conversation so everyone can see, along with a link for them to tap and change to another emoji.
- Unlike the color change, the new emoji will still appear when you use Facebook on the website.