As you have php 8.0 installed, when you update your composer.json file it have a constraint about php version on it.
Search for a line like this on composer.json
"require": {
"php": "7.0",
and change it to:
"require": {
"php": "8.0",
Then run composer update before installing new packages. There might be some breaking changes, as third-party packages could have upgraded to a new major version and composer update could fail. You should manually review those when upgrading PHP versions in a project.
If you are pretty sure your project will run fine in PHP 8.0 (I wouldn't be so sure as third-party code can have breaking changes from version to version) you can also try running composer install with this flag: --ignore-platform-reqs, for example:
composer require optimistdigital/nova-color-field --ignore-platform-reqs
Lastly, if you don't feel confident on upgrading the project from a newer PHP version you could downgrade your local PHP installation, or have a second PHP installation just for this project.