In the "review > Tools" page (available only to users with at least 10000 reputation points), in the list of posts to delete/undelete, some titles are displayed in a light color and I cannot cast votes upon them. What criteria do I have to meet in order to be able to do it?
Assuming that you have not used up your daily delete-vote allowance, it may not be a criteria that you as a user have to meet, but a criteria that the post has to meet. Here are some restrictions for applying delete votes.
- Users with between 10,000 and 19,999 reputation can only vote to delete closed/on-hold questions which have been closed/on-hold for at least two days.
- Users with ≥20,000 reputation can vote to delete closed/on-hold questions of score ≤-3 at any time, but still have to wait to vote to delete higher scored closed/on-hold questions.
- Users with ≥20,000 reputation can vote to delete negatively scored answers.
Being reopened does not remove delete votes on a question, but and does prevent regular users from casting further delete votes. An answer of score -1 being up-voted does not remove pending delete votes on an answer, but does prevent regular users from casting further delete votes.
I guess technically speaking a criteria that you as a user could meet to remove these restrictions is to be either elected as a site moderator, or hired by SE as a CM. Diamond encrusted users have no limitations on casting delete votes.