I have already used your ItemFormatting event in my code and implemented it the same way you have provided.
In my code, i have created 7-8 messages and fixed the first msg's timestamp to "6/6/2019 11:53:08 AM" and the remaining messages with timestamp of 2 minutes more than this time. Whenever the event is triggered i wrote a code which compares the timestamp of my message with this timestamp "6/6/2019 11:53:08 AM". Conditionally only the first message falls in this condition and i paint it red, which works well for first time (On load) and then when i scroll my chat, the event unnecessarily paints the remaining msgs too, which should not happen.
I am attaching my .cs file with designer please refer to it, and resolve this asap. Looking forward to your response.