In order to change color follow below-mentioned steps.
Login to CometChat Admin Panel.
Select "Appearance" from left side menu.
Add the new color and click on "Add Color" button.
You will see added color in left side section where we are displaying all colors.
Please click on one of the colors you added
Click on "Update Color" button.
Click on "Clear Cache" from left side menu
Clear Your Browser Cache
Refresh the page and check if selected color reflected for your "chat box".
We believe the above-mentioned steps were helpful to change color.