THE EYE HAS 7 MILLION light receptors, called cones, which respond best to red, green and blue. Hereditary colorblindness results from the absence of the proper pigments in some of the cones -- usually red or green ones.

A person in whom all three receptors work fine is called trichromatic; if one type of cone pigment is missing, he is dichromatic. If two are missing (very, very rare), he is monochromatic and lives in a world of shades of gray.

Except for monochromats, "colorblindness" is a misnomer; color-impaired or color- defective would be more accurate, since "colorblind" people can see most colors most of the time and might better be called "color confusers." Even trichromats can be partially "colorblind"; they have all three types of cones but the pigment in one cone is shifted slightly off the normal color response, usually on the red-green scale. Such persons are thus called anomalous trichromats and account for more than half of all colorblind people.

Among dichromats, there are two types of defect. A protanope is effectively red-defective. To him, red is darkened and often appears brown or black. According to color specialists Joel Pokorny and Vivian Smith, a protanope may confuse blue-green with gray and magenta, greenish blue with orange, light green with brown, dark blue with purple and yellow with bright red.

A deuteranope is green-defective. He may confuse blue-green with gray and with purple, green with yellow, orange or red, blue with blue purple and grass green with magenta. He also confuses pale shades -- greens and tans, blues and blue-purples, pale blue-green with gray or pale purple with gray.

While congenital colorblindness accounts for the vast majority of the defects, acquired or progressive colorblindness also occurs in some people, almost always as a result of a disease of the retina or optic nerve. Ophthalmologists use colorblindness tests on previously "normal" eyes to find symptoms of some eye diseases.

Hereditary colorblindness is incurable and untreatable. In recent years, a device called the X-Chrom has been used by some to heighten color distinctions. It is simply a red filter -- usually prescribed as a contact lens -- that is fitted over one eye. It blocks reds and darkens blues. This creates "retinal rivalry" -- the two eyes are rapidly viewing two slightly different images -- which heightens "binocular luster." It makes it possible to defeat the standard colorblindness test, the little set of dots with numbers hidden among the colors (the Ishihara pseudoisochromatic plate test).

But some scientists consider the X-Chrom dangerous, since it suggests improved color vision when this is scientifically not the case. The X-Chrom red lens has another drawback: a blue-eyed person looks pretty strange showin up with one red eye.