Some Tamyia colors are sold out

Actually Flat Black will do for new NATO Black. The color fades with age, so add a bit of dark gray to the black for a realistic weathered look. The real color has no green tinge.

Yeah, Tamiya USA shows it sold out:

https://www.tamiyausa.com/search/?q=nato+black

I agree with Tojo, supply chains.  Still disrupted as a result of the lockdowns.

If you're not wedded to Tamiya exclusively, it's a charcoal grey, so you might find an equivalent in some other brand's catalog.  I like to use Tamiya's NATO Black for a "scale" black-it looks like black in scale distance, eg, looking at the model on a display table, which would equal 40 or 50 feet in 1/1 scale.  But I can go to some other brands line Andrea or Vallejo (Model Color), or the craft store brands like Folk Art, Apple Barrel, Americana, etc, in a pinch.

Because... The Germans just voted to increase their defense budget, and so they have to repaint their Leopards as they have become quite shabby?

Awright, awright, I shaddap now.

In a pinch, add some dark green to black maybe?  I'll need some to paint the nori on my salmon roe sushi...

Supply chain issues