The translation must be modified from the template files. You just have to look carefully.
I do not have advanced experience in web development, but from what I noticed that text box cannot be modified. If I'm wrong, please let me know
For first time you need to login in StaffCP and find Groups button in menu. Create a new group and find "Group Username Colour".
From here, you can select what color you want using HEX codes.
I hope you found these instructions useful.
HTTP ERROR 500 is from your PHP version. Make sure you have selected PHP 7.4 or newest.
Yes. I use NamelessMC v2.0-pr12 and I have tha latest version of Lithium template by Xemah. I will try to reinstall template.