PhotoMaster,
Pat625 and
Issabella - thank you for your kind words.
Well Issabella what a tough task you set me
Here is a hopefully short treatise on the workflow I have used over the last couple of days in seeking various darker glass solutions -
1. In all cases I was using JWildfire fractals as my backgrounds
2. In all cases I used GMIC 3D Image Object to create the cup images from that background
3. In all cases I then created a seamless blended layer of that cup using GMIC>Layers>blend seamless
4. In all cases I used the worklfow from the original clear glass tutorial except that I did separate selections for the visible cup interior to stretch it and make it more natural looking when the glass layer was applied; I also frequently deleted the glass interior layer below the copied background layer
5. Then I started playing with adding the seamless layer, adding the original cup layer, adding a copy of the original cup layer having done a colour invert on it
6. Then I started playing with the blend modes and opacity levels all over the place
So as a result in some cases I got what I wanted but in others I did not
However, in summing up I think that just applying to a 'selection' of the glass object, a colour/gradient layer in 'overlay' mode on top of all the glass layers will give you darker or coloured glass to your liking ... well I hope so anyway.
Here are some samples of my travails ...
... but I had a lot of fun too.