As others have stated, you can't know what colors the grey was originally supposed to be. But when I read your question I thought you were asking for how to perform some kind of colorwash. Like to see instead of greyscale to have it in a yellowscale or something. If that is the case try researching with the key word colorwash. I found some articles about the process on
google[
^] including
this CP Article[
^].