To change Footer background color on One Page on Squarespace, you can edit page > Add a Block > Choose Code > Paste the code.
If the site support Code Injection, you can add it to Individual Page Header Code Injection
<!-- Footer background color - One Page -->
<style>
footer.sections {
background-color: #f1f;
}
footer.sections .section-background, footer.sections .section-border {
background-color: transparent !important;
}
</style>
