I do it by removing v-btn--disabled and playing with vuetify's css classes. 
Still grey but with colored text solution
The button will still be grey, but text will be colored, like that you have a visual effect showing that the button is disabled but still have a colored part.
I, personally, also had some custom opacity to disabled buttons. 
HTML
<v-btn id="btnA" :disabled="true" color="success">Success</v-btn>
CSS
button.v-btn[disabled] {
  opacity: 0.6;
}
JS
created(){
    // Trick to remove class after initialising form
    this.$nextTick(() => {
        document.getElementById('btnA').classList.remove('v-btn--disabled')      
    })
}
CodePen
Same display solution
If you really want, the same display you will have to remove [color]--text and add [color] class (and sometimes add white--text class for readability).
JS
created(){
    // Trick to remove class after initialising form
    this.$nextTick(() => {
        document.getElementById('btnA').classList.remove('v-btn--disabled')
        document.getElementById('btnA').classList.remove('success--text')
        document.getElementById('btnA').classList.add('success')
    })
}
CodePen