Hi,

Please refer the code below to change the label of checkbox to grey when it is disabled.

<div>

    <label for="checkbox_field" string="ssss" style="color:black;"

       attrs="{'invisible': [('checkbox_field', '=', False)]}"/>

<label for="checkbox_field" string="ssss" style="color:grey;"

       attrs="{'invisible': [('checkbox_field', '=', True)]}"/>

<field name="checkbox_field" nolabel="1"/>

</div>

Regards