In the question list we see that there are some questions which are pink. What is the reason of this situation?
The questions which are in different color are those ones, which have one of your favorite tags.
- Questions with these tags become highlighted on the front page.
- Questions with new activity are shown on stackexchange.com/filters.
- In the review queues, questions from these tags are shown to you before other questions.
The actual color is different for various sites - it depends on the design and color scheme of the actual site. (For example, they are highlighted with different color on Mathematics than on MathOverflow. The highlighting can differ even between main site and meta - for example, if you have favorited some tags here on MathOverflow Meta you can see that they get grey-ish background color.)
Whenever you happen to have the privilege to see deleted questions or answers, they are given a reddish background to differentiate them from the rest.