Ahhh thank you for that link I have done this before and I couldn't remember how!!! Though I had something cool happen:
I had my artist deliver my tiles as individual sprites not from a sprite sheet. creating a collection required no sprite sheet first I could just drag all the tiles in there. Was a bit quicker than creating a sprite sheet.
I did use the multiple materials way, was changing the texture more efficient? I wasn't sure how to do that, unless I have to do that in a separate program but it didn't seem that unity gives me any options to adjust the alpha except for a alpha from greyscale in an advanced texture type, is that what you were meaning?