HI there
Open your documents folder and click "ETS2MP". Then inside that should be a config.txt file - open that with Notepad.
In there is a list of all the settings for TMP and under "generals" you will find "tag_color" and then a 7 digit number. This is a decimal color. Go to this website https://convertingcolors.com/ and type in your RGB / Hex code and convert it to Decimal.
Once you have your Decimal color, just replace the number in your config.txt file and save it. Then you're good to go with your new tag color