I made a real one, in fact, a real big one, and that took quite a bit of effort with a very expensive industrial spectrophotometer. First of all, I took spectral readings of a huge stack of just about every kind of gray card you can think of that's sold in typical photo stores. Not a single one was reasonably close to 18% gray, nor were any truly neutral gray, and they were rarely consistent even within the same brand or specific product number. Things have improved somewhat since then, but I still distrust most reference grays except the gray patches on clean unfaded MacBeth Color Checker charts. So back then I batched up a quart of paint almost perfectly 18% reflective, truly neutral gray over the entire visible spectrum plus one step into both UV and IR. The process was too intricate to publish a formula; and it was basically a "what if" rainy day fun project. It could be mass-produced in a standard industrial batch, which is 144 gallon minimum, but nobody is going to do that because the quality control step afterwards would be too fussy. No can of paint one can buy in a store is even going to be close. Who ya kidding? You don't even begin to understand the manufacturing variables. I won't go into all the reasons why, but can start by stating that no black pigment is neutral. Every one of them has some kind of distinct bias; and it take a lot of work to fine tune that. If you take deep mineral black versus lampblack, for example, then lighten them, you'll discover a greenish bias in one, and a violet bias in the other. Commercial grays vary somewhat batch to batch, sometimes badly. And for an accurate reading, the specific color of the card does matter. Otherwise it will absorb rather than reflect certain wavelengths that you meter might be responsive to. You need a predictable reference, even when scene lighting itself varies. The less variables, the better. And gray cards are "supposed to be" a reliable standard, just as the receptor in you light meter is supposed to be. But I only use a true reference gray for critical color film tests at ideal color temperature, or for things like copystand lighting calibration. In the field, I'm comfortable aiming the spotmeter at a granite rock or road asphalt, or even a green lawn, and realizing from sheer experience what to expect.... Like I said, it was a project just for sake of a project.
Paint is a good idea. But you have to be careful. One day I got pigment and white and mixed it myself to make a two-tone gray target (Minor White), recently I just wanted to make a backdrop so I got a quart and told them to make it black, and another quart white. Mixed together it turned murky green.
