There are food colouring liquids, often used for cakes and stuff, but I don't know how this reacts to glass when you mix it with water. I guess you shouldn't leave it in for a long time, but taking a few pictures shouldn't take long … perhaps try it on a drinking glass first to see if it cleans up alright without leaving any traces.
Just to illustrate, I have just found the 1950 catalogue image from the Malmaison vanity set, I had missed it earlier on.