What do the colors on Stack Overflow mean?
In Stack Overflow I can see that some of the questions are shown in a different colour. What is meaning of that?
It means you've marked the tags in that question as your favorite..
The questions having your favorite tags appear in yellow background.
Check the top right part of the page to find your favorite tags
You need to define what you mean by a different colour.
If you mean that questions can have a yellow background colour, those are tagged with one of your favourite tags. If they have a dimmed, greyed out appearance, they have tags you have added to your ignore list.
But if you are talking about the vote count on the left, then you see differences based on whether or not there are answers for that question (green background), and if one of those answers has been marked as accepted (yellow answer count instead of white).