@Ewdy

Another way of doint this dynamically is to add decoration attribute which can take a condition and changes color based on it.
Please see below example of how to add it to a field in xml

<field name="expected_revenue" widget="monetary"
options="{'currency_field': 'company_currency'}"
decoration-success="expected_revenue &gt; 0" decoration-danger="expected_revenue &lt;= 0"/>

To find more about how to work with decoration, Please check the link below:
https://www.odoo.com/documentation/master/developer/reference/backend/views.html?highlight=decoration