The questions which are in different color are those ones, which have one of your [watched tags](https://meta.mathoverflow.net/tags/tag-watching/info). (This feature used to be called *favorited tags*, but it [was later renamed](https://meta.stackexchange.com/questions/312180/favorite-tags-is-now-tag-watching).)

From [What do “watched tags” and “ignored tags” do?](https://meta.stackexchange.com/q/19173)

> * Questions with these tags become highlighted on the front page.
* Questions with new activity are shown on [stackexchange.com/filters](//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](https://math.stackexchange.com/questions) than on [MathOverflow](https://mathoverflow.net/questions). The highlighting can differ even between main site and meta - for example, if you have some watched tags [here on MathOverflow Meta](https://meta.mathoverflow.net/questions) you can see that they get grey-ish background color.)

On MO, it looks something like this:

[![screenshot from the main site][1]][1]

And this is from Meta MathOverflow:

[![screenshot from meta][2]][2]


  [1]: https://i.sstatic.net/nDFCk.png
  [2]: https://i.sstatic.net/tjO1Z.png