I never read a precise definition of “render” for comic book coloring, and I heard several people struggling to get the concept of it, so I came up with one myself: RENDER is the amount of COLOR INFORMATION used to complement a black and white piece of comic book art.
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What do I mean with "color information"? Well, turns out that color as we know it has three attributes we can manipulate: HUE, SATURATUION and VALUE (brightness).

HUE will tell you what color you're looking at. Is Nightcrawler'skin blue? Or is it green? Maybe red? If you know the answer, you know your hues!

SATURATION will tell you how strong and vibrating that color is. Is it a jarring pink or a soft shade of pink, like a picture taken long ago?

VALUE will tell you if something is bright or dark. Is Nightcrawler hard to spot because everything is so dim? Your values are too dark. Can't you tell the difference between the blue from the beard and the flesh because everything looks pale? Your values are too light.

So, basically, to RENDER is to choose which COLOR INFORMATION you include in the B&W art and which you leave out.

Mostly of the colored comic book art we see is done with the variation of all three aspects of color (hue, saturation and value) to some degree. But it is possible to render an image while varying just two or even one attribute, wanna see?

If we leave HUE and SATURATION out, we can RENDER the art only with grayscale values. And Nightcrawler will look like first pic. Or maybe like pic two? The possibilities are almost endless.

On the other hand, if we use just ONE VALUE and keep our hues diverse, we can make something like the first picture. These are both the same, but second one is in grayscale. Notice that our saturation is almost the same in the whole image.

And the same thing goes with SATURATION. While our VALUES are very similar in this picture, we have different saturations for different parts of the art. Nightcrawler skin, hair and red clothes are much more saturated than the background and the eyes.

So RENDER is the amount of color information inserted in a originally black and white drawing. And we can say that a heavily rendered art is an art with a lot of color information. Logically, a light rendered art would be one with little color information.

I don’t thik we can really set or measure the maximum of RENDER we can insert in one piece. Maybe is possible, mathematically, but we can definitely set the minimun with just logic: a flat, singular color. One hue, one value, one saturation!

And that's basically it, guys. I went with extreme examples here to make a point, but hopefully this will clear some of your doubts about rendering in comics. Oh, and this gorgeous drawing was done by the amazing @ArtofNickRobles!

PS: I thought about using the term COLOR VARIATION instead of COLOR INFORMATION in this definition, but I think that would be wrong. Because if we accept that “no saturation” exists, we no longer have hues, which means no variation in colors. Only values. And you can certainly

...render something with values only. Finally, a singular flat color being considered render is weird to me too, I didn’t antecipate that, but if we accept the term COLOR INFORMATION, than we absolutely have information from just one simple color!