Moderator edit: please use code blocks when posting configuration files

[
{
"GroupName": "default",
"Priority": 2,
"Title": {
"Text": "[Player]",
"Color": "#55aaff",
"Size": 15,
"Hidden": false,
"HiddenIfNotPrimary": false
},
"Username": {
"Color": "#55aaff",
"Size": 15
},
"Message": {
"Color": "white",
"Size": 15
},
"Format": {
"Chat": "{Title} {Username}: {Message}",
"Console": "{Title} {Username}: {Message}"
}
},
{
"GroupName": "admin",
"Priority": 0,
"Title": {
"Text": "[Admin]",
"Color": "red",
"Size": 15,
"Hidden": true,
"HiddenIfNotPrimary": false
},
"Username": {
"Color": "red",
"Size": 15
},
"Message": {
"Color": "white",
"Size": 15
},
"Format": {
"Chat": "{Title} {Username}: {Message}",
"Console": "{Title} {Username}: {Message}"
}
},
{
"GroupName": "mod",
"Priority": 1,
"Title": {
"Text": "[Mod]",
"Color": "purple",
"Size": 15,
"Hidden": true,
"HiddenIfNotPrimary": false
},
"Username": {
"Color": "purple",
"Size": 15
},
"Message": {
"Color": "white",
"Size": 15
},
{
"GroupName": "donator",
"Priority": 2,
"Title": {
"Text": "[Donator]",
"Color": "#FFFF00",
"Size": 15,
"Hidden": false,
"HiddenIfNotPrimary": false
},
"Username": {
"Color": "#FFFF00",
"Size": 15
},
"Message": {
"Color": "white",
"Size": 15
},
"Format": {
"Chat": "{Title} {Username}: {Message}",
"Console": "{Title} {Username}: {Message}"
}
},
"Format": {
"Chat": "{Title} {Username}: {Message}",
"Console": "{Title} {Username}: {Message}"
}
}
]​

Changed the Priority of default to 3 being the highest.

Merged post

Changed the "HiddenIfNotPrimary": to TRUE.

You seem to have moved the "mod" group "Format" section to the end of your file.
To verify you haven't made any syntax errors you can use http://jsonlint.com

Your groups file should look something like this: https://pastebin.com/zAim9SQz

It looks like your priorities are configured correctly though.

If you're not comfortable editing the group file manually I recommend you either use the commands or the Better Chat Group Manager to make changes. The Better Chat Group Manager is linked on the top of the plugin page.