If you will look through the red glass on the inscription made in red and white, you'll see smooth red background, and only. No trace of the labels you will not notice, because red letters merge with the same red background. Looking through the same glass on the inscription, blue and white, you'll clearly see black letters on a red background. Why black is easy to understand: red glass does not transmit blue light (because it is red, which passes only the red rays); therefore, in place of the blue letters you should see the absence of light, i.e., black letters.
This property of colored glass based action of the so-called of anaglyphs - paintings, printed in a special way and gives the same effect as stereoscopic pictures. In anaglyphs both images corresponding to right and left eye, are printed on top of one another, but different colors: blue and red.
To see instead of two colors, one is black, but the relief image, it is enough to look at them through colored glasses. The right eye through the red glass sees only blue imprint, i.e. the one that corresponds to the right eye (and it seems the eye is not a colour, and black). The left eye through the blue glass only sees the corresponding red imprint. Each eye sees only one image that corresponds to it. We have here the same conditions as in the stereoscope, and therefore, the result should be the same: it creates an impression of relief.