Gray cards are used because they are neutral: they reflect roughly the same proportion of red, green, and blue light. That makes them a stable reference for exposure under different light sources.

A colored card is not neutral. An “18% red” card would reflect red much more strongly than green or blue. Under lighting with plenty of red, it could look relatively bright; under cooler light with less red, it could look darker. So its apparent brightness changes with the color of the illumination, making it unreliable as a metering standard.

An 18% gray card, by contrast, is intended to reflect about 18% of the light across the visible spectrum rather than favoring one color. That’s why it works better regardless of whether the light is daylight, shade, or tungsten.

So the key point is: for metering, neutrality matters more than just the total reflected light. Gray is used because it gives a consistent exposure reference without being biased by the scene’s color temperature.