The questions which are in different color are those ones, which have one of your watched tags. (This feature used to be called favorited tags, but it was later renamed.)
From What do “watched tags” and “ignored tags” do?
- 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.)