How can i turn a greyscale image into a black to transparent image?

I once made a jaket that had multiple colours into one color, for this i somehow turned a greyscale image of this jacket into an image where the Image nolonger was greyscale but from black to transparent, then i easily layed a colour a level beneath this and the jacket now was one colour. does someone know how you can thurn a greyscale image into a black to transparent image?