In your customer contacts section, you may notice that the toggles for some contacts in the "Subscribed" column are a darker gray color than others.
Darker gray toggles show contacts that you or another user on your account have designated as "Not Subscribed" so that they will not receive any marketing messages. These contacts can be re-subscribed by clicking on the toggle to turn it purple.
Lighter gray toggles show contacts that have "Unsubscribed" themselves from your marketing messages. These contacts cannot be re-subscribed.