Shukan Flavor No. 62 is about nuru kan coffee.
Shukan Flavor No. 62 Thinking of Nuru Kan Coffee/週間フレーバーNo. 62 ぬるかんを考える
He also discusses nuru kan coffee here (Japanese only) on the official website of Flavor Coffee.
From here on, Japanese text omitted/ここより、和文省略
On the website, he argues: Sake (Japanese rice wine) can be drunk hot, cold, or lukewarm, but coffee is drunk either hot or cold. As time passes, all coffee will approach room temperature. There may be such coffee that will taste good then.
He calls such coffee "nuru kan" coffee.
Note: The term nuru kan was not coined by Nakagawa-san. Hot sake is called atsu kan, cold sake hiya zake (or simply hiya), and lukewarm sake nuru kan.
Strictly speaking, nuru kan is sake at temperature of about 40C, but Nakagawa-san says that nurukan coffee is coffee that tastes good at room temperature.
He makes nuru kan coffee by mixing acid light roast coffee with bitter dark roast coffee at an appropirate ratio. He says that nuru kan coffee is best suited for driving.
In the Shukan Flavor video above, Nakagawa-san says that acidity and bitterness cancel each other out, and suggests mixing light roast coffee with dark roast coffee at a ratio of 1:1.
In the video, he also makes nuru kan coffee by mixing cold-brewed light roast coffee with cold-brewed dark roast coffee. As for cold-brew coffee, he suggests a light roast to dark roast coffee ratio of 3:1.
He describes nuru kan as "tea-like" and "near-water".
I think nuru kan coffee is another great idea of Nakagawa-san, but I haven't tried it because I believe that coffee tastes best when brewed hot.