In the contest tab, only the questions you solved during the contest are marked green. If you upsolved or solved it in virtual contest the question is not marked green. I don't know what's the logic behind this. Marking questions green helps a lot while upsolving or while deciding which contest to give as a virtual contest. Of course you could go to the submissions tab and then see whether you have solved it previously or not but I consider it an overkill. Codeforces marks the question green irrespective of whether you solved it during the contest or during upsolving and you can also learn from them. I don't think anything would be done but still it's irritating me for a lot so I decided to write about it.
Peace!