Inspired by this topic, and the custom colors of Discord roles, I have written this very simple code that adds a data-mention to the mentions of groups and users, in this way to be able to place different colors (And not “general color”) via CSS 
The code at the moment is JQuery but at some point I will change it to Vanilla JavaScript 
To apply it you just have to paste the code in the head of your theme or component:
<script type="text/discourse-plugin" version="0.9">
$(document).ajaxSuccess(() =>{
$('a.mention').each(function() {
$(this).attr('data-mention', $(this).text().replace('@', ''));
});
$('a.mention-group').each(function() {
$(this).attr('data-mention', $(this).text().replace('@', ''));
});
});
</script>
After this you can apply the style you want through CSS 
For example:
a.mention[data-mention="Alexander"] {
color: #4527a0 !important;
}
In this case it was for a specific username, however it also works for groups using a.mention-group[data-mention="group-name"]
I’ll look into making this a Theme component that can be customized from its “settings” without modifying the CSS 