Take a picture of the words on the white paper with a camera. Why is white paper always gray?

Because the metering standard of the camera metering device is based on 18% gray scale!

Therefore, the photometric device will "translate" all objects into the gray scale of 18, and then calculate the aperture and shutter value (EV value) required for accurate exposure.

If you aim the lens at a large area of black, black is also "gray".

Metering equipment can be roughly divided into reflective and incident types, and now the built-in cameras are reflective metering; Hand-held exposure meter has the functions of reflection and incidence, and can also measure electronic flash. The incident type is to measure the light received by the surface of the object, regardless of the reflective factor of the object, that is, whether the object is black or white, the measured values are the same, and the effect of the photo is close to the actual situation; Reflective measurement is to measure the light reflected by the object to the exposure meter (the measurement benchmark is 18 gray). Different black and white reflectivity requires different exposure value. For example, the exposure time required to shoot a white object will be shorter, so that the result of the photo will be closer to 18 gray, and sometimes the light and color will be inaccurate. In order to accurately restore the color and brightness of the object, it is necessary to use the exposure compensation function of the camera.

In addition, color photography involves the problem of color temperature, that is, the white balance of digital photography now mainly affects the hue deviation of photos, that is, people usually call it color cast. But your problem is mainly the exposure reference value, not the color temperature (white balance).