Added in 1.3.13, tried my best to let you customize your rcon chat message format but if you use RustAdmin this might not work for you. You can change the layout now but it will only show up in the 'Console'-Tab of RustAdmin if you do so. Seems to be especially listening to the default format so I would suggest to not change the option in the localization file rather leave it as default and keep also getting the messages into the 'Chat'-Tab.

Another thing is that RustAdmin doesn't show Team messages in the 'Chat'-Tab, same reasoning hear I believe they would need to update their client to fix this.

I messaged the dev of RustAdmin we might have a chat and could figure something out here that would also profit you in this case!

(16:47:41) | [CHAT] RconLogFormat : test
(16:47:41) | {
  "Channel": 0,
  "Message": "test",
  "UserId": "number",
  "Username": "mr meeseeks",
  "Color": "<color=#FF00F2>m</color><color=#E619F3>r</color><color=#CC31F5> </color><color=#B24AF6>m</color><color=#9963F7>e</color><color=#807CF8>e</color><color=#6694FA>s</color><color=#4CADFB>e</color><color=#33C6FC>e</color><color=#19DEFE>k</color><color=#00F7FF>s</color>",
  "Time": 1577375244
}



(16:48:05) | [CHAT] Marty[number] : test
(16:48:05) | {
  "Channel": 0,
  "Message": "test",
  "UserId": "number",
  "Username": "Marty",
  "Color": "<color=#FF0000>M</color><color=#FF4000>a</color><color=#FF8000>r</color><color=#FFBF00>t</color><color=#FFFF00>y</color>",
  "Time": 1577375268
}

Meeseeks vs Marty(me)

Mine is shown in the Chat-Tab every time, but Meeseeks chats never show up :(
(Doesn't matter if he uses gradient, or single color, or if we use the exakt same setting. He is invisible when he is using a color.)

The only thing that is suspicious is the difference between meeseeks and me:
meeseeks

(16:47:41) | [CHAT] RconLogFormat : test

Marty

(16:48:05) | [CHAT] Marty[number..] : test

Puts("DebugPuts - '" + player.IPlayer + "'");
Puts("DebugPuts - '" + player.displayName.EscapeRichText() + "'");
string formattedMsg = GetMessage("RconLogFormat", player.IPlayer, player.displayName.EscapeRichText(), player.UserIDString);
Puts("DebugPuts - '" + formattedMsg + "'");​

Result in RustAdmin-Console-Tab:

(17:06:56) | [Coloured Names] DebugPuts - 'Covalence.RustPlayer[number, Marty]'
(17:06:56) | [Coloured Names] DebugPuts - 'Marty'
(17:06:56) | [Coloured Names] DebugPuts - 'Marty[number]'
(17:06:56) | [TEAM CHAT] Marty[number] : test


//VS

(17:07:01) | [Coloured Names] DebugPuts - 'Covalence.RustPlayer[number, mr meeseeks]'
(17:07:01) | [Coloured Names] DebugPuts - 'mr meeseeks'
(17:07:01) | [Coloured Names] DebugPuts - 'RconLogFormat'
(17:07:01) | [TEAM CHAT] RconLogFormat : test

Somehow your function "GetMessage()" loses "player.IPlayer" and "player.displayName.EscapeRichText()" arguments.
Maybe you can debug it to verify.