I'm thinking maybe Diamine Twilight Blue or even Eau De Nil could be a reasonable substitute for Tsuki-yo.
What are you thoughts? Have any of you used or use a good substitute colour ink instead of Tsuki-yo?
I'm addicted to it, It's an incredible ink and colour. I'm not saying I use a lot because i don't. In fact I bought a bottle in July 2012 and I still have half of it left and I write in my journal every day now. (Of course thats mixed colours between MB Royal Blue, MB Irish Green and Waterman Florida Blue)