These little "here's how it would look" tools work well enough for sanity-checking images on a computer monitor, but don't necessarily tell you much about what someone will actually see. Color blindness makes a lot more sense if you have a decent grasp on how human color vision <i>actually</i> works.<p>Quick summary: Most people have four different types of light receptors. One type responds to all colors, the other three have overlapping response curves covering parts of the visible spectrum. Perception of hue and saturation derives from the <i>differences</i> between how strongly the three color receptors react--even "pure" colors will usually trigger at least two color receptors. Because of this, for any given triple of receptor response strengths, there are many combinations of light that will cause it; the RGB color system exploits this, using three colors in varying proportions to create a large range of perceived colors.<p>Colorblindness is a catchall term for defects in one or more type of light receptor. Which type, and the nature of the defect, can cause widely divergent effects.<p>Some people have anomalous response from one type; i.e., a nonstandard response curve. In practice, this means that their color vision essentially works like anyone else's, but the set of light combinations that cause a given response differ, so two colors that look identical to you may look different to them! For instance, if the medium-wavelength receptor's curve is shifted downward to further overlap the long-wavelength receptor's curve, reddish colors will seem more green and bluish colors will seem less green. Compared to a normal spectrum, this makes the cyan range shorter and the yellow range larger, making it harder for the individual to distinguish between mixtures of red and green.<p>Other people lack one color receptor type <i>entirely</i>, and thus have only two-axis color perception instead of the normal three. Losing the long or short receptors makes extreme reds or blues effectively invisible, while losing the medium-wavelength receptors leaves the individual with a full range of sensitivity, but able to perceive only a single gradient between colors. Note also that this collapses hue and saturation into a single axis; for instance, to someone with only the long and short receptors, there are both purples and greens that will look like white light.<p>A very few people have only one type of color receptor, or none at all, and thus perceive only overall brightness, with no color at all.