There are 2 different ranges of paints from this company. Mr colour & gunze sanyo (aquieous hobby colour). Both have their own thinner. Mr colour is solvent based (similar to tamiyas acylic range) & gunze is water based.
I use the gunze range a lot along with tamiya. I have never used water to thin any paint. Metho is great for clean up for all these paints in the airbrush and grubby paint brushes. You can thin with metho but it gives you a very flat finish. I would not reccomend it.