In the question list we see that there are some questions which are pink. What is the reason of this situation?
- 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 some watched tags here on MathOverflow Meta you can see that they get grey-ish background color.)
On MO, it looks something like this:
And this is from Meta MathOverflow: