Hi Kamylle,
Welcome to the forums
This permission is set through Mac OS, meaning you'll need to change the setting outside of the Affinity app.
Please open System Preferences > Security & Privacy then select the Privacy tab. Click on the lock to make changes (you may need to enter your Mac password here), then tick Affinity Designer in the list of apps here. Now you can press the Lock icon again to save changes, restart Affinity Designer & you should find the colour picker is now working as expected!
