Placeholder text in dropdown menu

I’ve set the first item in my dropdown menu to not be a valid choice, but I was wondering how I would go about setting the colour of only the first item to be grey to match the other placeholder text throughout the form.

At the moment I’m using this css which I found in an old topic here, but it changes the colour of every item in the list, how would I go about targeting only the first one?:

#lp-pom-form-206 .lp-pom-form-field select
{
border-radius: 5px !important;
color: #a9a9b0 !important;
}

#lp-pom-form-206 .lp-pom-form-field option
{
color: #000 !important;
}