Here we're talking specifically about "visual contrast", which has a meaning in perception slightly different than the common usage of "contrast".
Visual contrast is measured on a spectrum that starts at zero and goes up from there - a box of a particular color inside a box of the same color would have no contrast, but if there is any difference in the color values then there is at least some visual contrast.
Wikipedia isn't a great source for definitions, but has a reasonably thorough take on this https://en.wikipedia.org/wiki/Contrast_(vision), going into the details of contrast sensitivity, which deals with almost-imperceptible levels of contrast.
Visual contrast is measured on a spectrum that starts at zero and goes up from there - a box of a particular color inside a box of the same color would have no contrast, but if there is any difference in the color values then there is at least some visual contrast.
Wikipedia isn't a great source for definitions, but has a reasonably thorough take on this https://en.wikipedia.org/wiki/Contrast_(vision), going into the details of contrast sensitivity, which deals with almost-imperceptible levels of contrast.