Nice edition of the gaff as always!
The labs idea about receipts that you have highlighted, does that refer to printing out purchase confirmations ect on the giffgaff website or till receipts when topping up in a shop?
If shop receipts then this idea will not actually have the effect the creator is after, I appreciate what they are trying to do to be green however the type of paper that is used to print receipts is thermal paper, thermal paper already contains the ink, and the printer heats it up in the area the text or negative space should be, which turns it black, so the paper is already fully covered in ink and the printer in-fact contains none. You can test this yourself by placing a receipt on top of a warm surface (hob ect) for just a few seconds and it will turn fully black.
So unfortunately not printing as much black will in-fact male no difference in this case I'm afraid. thermal paper unfortunately isn't recyclable, so any and all receipts are a waste, the good news though is lots of companies are moving to digital receipts and soon this problem will hopefully be no more.
And if they mean from online on the giffgaff website, then I'd probably just save it on the computer/backup ect and save the process of printing everything to put in a file. I'd imagine there is a record kept on the website anyway for a reasonable timeframe.