The questions which are in different color are those ones, which have one of your [favorite tags](https://meta.mathoverflow.net/tags/favorites/info).

From [What do “favorite 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/) than on [MathOverflow](https://mathoverflow.net/).