Hi Jamie,
You can achieve this using dynamic classes. You can find this under the tab appearance of the combobox properties. Using an if statement like this:
if $currentObject/YourAttribute = empty then 'customcomboboxempty' else empty
And add this custom css to your styling
.customcomboboxempty .form-control {
background-color: aliceblue;
}
This way the combobox is aliceblue when not filled or focused.