Its funny. I was about to post an update! I didn't yet try the charcoal idea from @Berties but I tried just about everything else. If you are into grey pasta, I'm now an expert....how is the quest coming?
However, I was going to post because someone has beaten me to it. Quite by chance I found and ordered this from Ocado. Its made from black beans which is one of my ideas and which I tried out. However, this is made entirely from black beans (no flour), hence the black, rather than grey colour. Arguably its not really pasta! Its a Swedish product and is called
A La Eco Black Bean Spaghetti. Anyway, I bought some as it was on offer for £1,99. I'll report back on whether it tastes anything like pasta.