Black only indicates that this resource has been found and can be downloaded, but the number of available sources is very small (generally less than 10).
The green one has been downloaded, or you chose to download it, but later you manually deleted the download task, which means that you have finished processing this file.
Red means downloading, just click Download.
After restarting, the green and red files will remain unchanged, while the black and blue files will be identified according to the number of available sources in the current search. It is possible to search for black in the morning and blue at night when there are many people. In addition, if the file has been downloaded or is being downloaded, when you click Download again, you will be prompted that the file has been downloaded or it already exists in the download list.