Hi Vij,
A note with gray icon specifies that it is obsolete for the system you are upgrading. The system is already in higher release than the release for which note is applicable.
Note with Yellow icon needs to be checked if it can be applicable to the system. SNOTE or service marketplace is where you can see releases for which note is applicable and check if your system release is listed in it.
Note with '?' says that system could not determine the adjustment mode of it. You can re-determine the status of it by clicking on it. A dialog box appears asking to run background job to determine the adjustment mode.
There is color legend available which tells what each color specifies about adjustment mode of the object.
Hope this helps,
~Athreya