Like everything else, lets begin from basics
What is color?
Most of us will understand the definition: A visual attribute of things that results from the frequency of light they emit or transmit or reflect. So it is the frequency of light that gives a stimuli to our retina(to the conical receptors in retina, to be precise), and we see the respective color.
Now, it is an amazing fact that explanation to something as artistic as color lies in a thing as technical as frequency of light wave, but at the same time it is also a reason the explanation becomes tricky for a layman to understand. But lets proceed with whatever it takes.
Corresponding to the three primary colors red, green and blue there are three types of conical receptors each sensitive to the stimuli of the respective color. The other colors which are composition of these, say Purple, stimulate the Red and Blue cones simultaneously and a new color is the response.
This however doesn't generally mean that Blue + Red = Purple. Consider this, we know that a purple bulb will emit light which has frequency same as that of purple. If I have two heaps of dust, one blue and one red and I mix them, anyone can guess that it will appear purple if mixed in significant proportions. Does it mean that the reflected light has same frequency as that of purple? Certainly not. The frequency of light in original dust (reflecting red and blue) will not change when we mix them. Why does it appear purple then? It is because the two types of cones respond to both the colors at "nearly" the same time and we perceive it as purple.
Whatever, looking at purple or mixing powders, if same composition gives the same color, whats the difference?
The difference is that it gives same color only for observers with trichromatic vision and have a visibility range between wavelength of Red and that of Violet and have normal color sensibility(Humans are the best example)
Trichromatic vision is one where the eye responds to stimuli of 3 colors. Humans are trichromatic, with rare exceptions. Some animals are dichromatic as well. Some rare primitives are monochromatic. All digital color display equipments are optimized to stimulate the senses in human eyes. They don't always emit purple light for a purple object, but often a mixture of blue and red light, which has the same effect on normal human eyes(like the dust mixture).
An experiment on chimps further explains this fact.
In Chimpanzee's cage was first put a mirror and it was later replaced by a TV displaying his own live feed, laterally inverted, at such an angle that it appears to be a mirror nonetheless. A human would have responded identically to both situations, but it was observed that the chimp was excited at seeing a chimp behind the mirror but later became normal when he understood that it is his own image, however it was particularly confused when placed in front of the TV.
The explanation lies in the fact that,composition of blue and red colored light did not stimulate the 'purple senses' in him. The TV was designed for human color spectrum vision and chimp was confused to see another chimp who looked different(different color) but copied his moves.
This is the place where color blindness best fits in.
Consider a situation where a person orders goods on phone. If both people on phone know the same language, they would work out fine and it will be called a boring office. If they don't, they will know it before starting the communication and they will hang up. Consider a rather hypothetical situation where the two people use two languages which are otherwise the same, the only difference being names of objects that are ordered. For example, iron block in one language would mean wooden block in another. This office, would land up in chaotic troubles every time and somebody will surely get fired in a long run.
Keeping aside the feasibility of having two such languages, lets look at the two people again. Both people see the same wooden block but call it different names, because to one person it simulates the senses that make him call it wood and to other it makes him call it iron.
What exactly is Color Blindness?
Color Blindness is of various kinds. When one cone system is compromised, dichromacy results. The most frequent forms of human color blindness result from problems with either the middle or long wavelength sensitive cone systems, and involve difficulties in discriminating reds, yellows, and greens from one another. They are collectively referred to as "red-green color blindness", though the term is an over-simplification and is somewhat misleading. Other forms of color blindness are much more rare. They include problems in discriminating blues from yellows, and the rarest forms of all, complete color blindness or monochromacy, where one cannot distinguish any color from grey, as in a black-and-white movie or photograph.
On the other hand Deuteranomaly aka Daltonism aka red-green color blindness is the most common type of color blindness(the same one that I have) where the observer still has trichromatic vision, but the medium-wavelength pigment(the cone sensetive to green) is shifted towards the red end of the spectrum resulting in a reduction in sensitivity to the green area of the spectrum. In other words, the green cone (which should actually respond to green) responds to a color which a normal person will say lies between green and red, thereby, reducing the response to green, according to the normal person. There are other but similar types of color blindness in trichromatic vision as well. This (so called) disorder is found on an average in 6% of males and only 0.4% of females.
The point is, Daltonism is not a lack of vision, it is actually a vision that is inconsistent with normal vision. I can give you two explanations for this.
1. Many people know that color blindness is tested using Ishihara Color Test, where a pattern of various dots of different color is to be recognized. Below is an example.
You will be surprised to know that this particular pattern, appearing '74' to normal eyes appears '21' to Daltonic eyes, which cannot be perceived by a normal eye.
2. There are some studies which conclude that color blind individuals are better at penetrating certain color camouflages and it has been suggested that this may be the evolutionary explanation for the surprisingly high frequency of congenital red-green colour blindness(Daltonism). These studies suggest that Anomalous Trichromats tend to learn to use texture and shape clues and so are often able to spot camouflage clothing, netting, and paint that has been designed to deceive individuals with color-normal vision.
Needless to say that Dichromism and Monochromism(as opposed to trichromism) are more severe forms of color Blindness
Let us have a look at what wikipedia thinks a disease and disorder is:
Disease (noun) (plural diseases): An abnormal condition of the body or mind that causes discomfort or dysfunction; distinct from injury in so far as the latter is usually instantaneously acquired.
Disorder (noun): a physical or psychical malfunction
Clearly Color Blindness is not a disease. What I want to assert is that calling Color Blindness a disorder is nothing but a careless categorization.
If I were to correct it I would say, "In general, Color Blindness is an anomalous vision which is inconsistent with normal vision. Of the various types of color blindness, Dichromism and Monochromism can be categorized as disorders."
Color Blindness is generally incurable. In some cases, it is even progressive. As progressive phenotypes involve deterioration of the retina and other parts of the eye, certain forms of color blindness can progress to legal blindness, i.e., an acuity of 6/60 or worse, and often leave a person with complete blindness. Thank god its is rare.
My worst reality with color blindness is the fact that as a kid I was good at fine arts but my interests were limited to pencil sketching, as combinations of colors used to confuse me a lot and even if I tried using them, the choice of colors I loved was not appreciated by others, as it was almost always inconsistent with choice of people with normal vision.
