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 the meta site][2]][2]

<hr>

After inspecting the site, I found `tagged-interesting{background-color:var(--yellow-050)}` both on the main site and on meta. You can see various colors used on Stack Exchange sites in the [Stacks Project - Colors](https://stackoverflow.design/product/base/colors). (It seems that people perceive colors differently. And it might also depend a bit on the setup of your monitor. The background color was described as *pink* in this question and as *yellow* in [another question here on meta](https://meta.mathoverflow.net/q/5072).

On Meta Stack Exchange you can find also some discussions related to changing the background color - however, they are rather old, so the answers might be outdated: [How can I change the CSS for a question that has a favorite tag?](https://meta.stackexchange.com/q/77181), [Define your own color of tagged-interesting](https://meta.stackexchange.com/q/66592), [Double-intensity shading for questions with >1 Interesting tags](https://meta.stackexchange.com/q/49704). There is also a similar post on Meta Stack Overflow: [The highlight colour for interested tags](https://meta.stackoverflow.com/q/345045). I will remind also a post on this meta about getting MO to look closer to "ye olde version": [How can I customize site in my browser to get “old feel” look of MathOverflow?](https://meta.mathoverflow.net/q/4022)

<hr>

If you're not sure where to find (or change) your watched tags, you can consult the [FAQ post mentioned above](https://meta.stackexchange.com/q/19173). I will add a link to my post on Mathematics Meta, which contains also some screenshots: [How do I follow tags?](https://math.meta.stackexchange.com/q/27863#27881)

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