I've read a fair bit, asked some questions and respected opinions I value. As far as colours go, they are indicators of a stones performance based on past traded similar types. I have honed for over 5 years before getting a Japanese natural stone. My opinion on coticules is my own, and I prefer not to use them, based on actual use of top graded stones. About cost. Well, like any other commodity they are what the market says they are. We have heard and seen many who swear by their sub $100 USD jnats, mostly referring to shoubu-dani stones. We all see the amazing prices asked for and paid for some stones. This is where size does matter. A full bench sized (say 8 x 3 x 1) in favoured mined stones, like your Nakayama and Ozuku will command the highest prices when you must then consider those 'colours' and there are many types to place them in the stratosphere on prices. Would I pay four grand for a stone? No, but there are many that will. For others who are considering Japanese natural stones, go in with your eyes open. You do to a large degree get what you pay for. I can only tell of my experience as the owner of 2 'jnats'. To me, I wanted some firm commitment by the vendor that I would receive what I asked for and expected, or they will accept it's return. That gives you confidence that for any such trader to make a living he has to be supplying decent stones. Don't buy without such guarantee. My 1st stone was a 212mm x 78mm x 38mm 1400 grams, a double sided Ozuku Asagi. I asked for a full sized stone, that would be a final stone, a finisher. I got exactly that, and in the 4 months since, so happy was I that I bought a second, another full sized double sided Wakasa, another hard stone. That'll take me to number 3 which is on the way. Same vendor. Guaranteed and tested on razors. The third not double sided and slightly smaller at 1100 grams. It's in the mail. My most expensive purchase was $455 usd, the cheaper being $325 USD. For what I actually received I am happy, and haven't sent any back. As far as what I paid, I believe it is low to middle ground on what to expect to pay. The two I have, have been photographed post lapping and are in the 'hones' pages on this forum with an opinion. I'll do the same after my third one arrives.
Go in with open eyes, buy from a trusted source, and learn what the price can get you, and , what you want. Good luck.