Metamer [met-uh-mer]
“ A color that appears to the eye to be identical to another color but which in fact has a different spectral composition.” Thank you dictionary.com
“Metamerism is a psychophysical phenomenon commonly defined as the situation when two samples match in color under one condition, but fail to match under another condition. The underlying basis of metamerism is that visual color matches are possible between two samples even though the spectral reflectance factors of the two samples are different. Thus many color matches are conditional. Conversely, if two samples have identical spectral reflectance factors (yeah, buddy. whatever. ed.) they unconditionally match when viewed under the same conditions.
Does your brain hurt yet? This is usually when we takes a lunch break - "What do you mean it doesn't MATCH?".
Observer metamerism occurs because of differences in color vision between observers. Often these differences have a biological source so that, for example, the ratio of long-wavelength-sensitive cones to medium-wavelength-sensitive cones may differ from one person to the next. Thus, two spectrally dissimilar surfaces may match for one observer when viewed under a certain light source but then fail to match when viewed by a second observer under the same light source”. Thank you wikipedia.com. Hey, it looked green to me. Maybe it's your eyes.
The dreaded metameric shift under different lighting conditions. Same targhee sample skein on a 50% grey background photographed with different white balances;

White Balance preset using the yarn as “white”

Fine setting

Fluorescent setting

Cloudy setting
And that is why, in 23 years I have run in to two (2, count ‘em) designers with whom I have shared the same sense of color perception. It is one heck of a ride to gleefully wing up to a designer with a sample swatch and say “Here, this is the color you want, right?” to then have them look at you as though your head self-clashes when that in fact is exactly what is happening - in their eyes. Like playing with the white balance on your digital camera. Except it’s coming straight down your optical nerve to your brain and is the only version of “correct’ you get to work with. Unless you add rose colored glasses and oh, la, it's the 70's all over again.
There are also the initial descriptions to get around “Oh, I want something in a sort of pastel tangerine with a toasted almond over wash.” Of course you do. Here’s the Pantone® book, you pick 3 chips and I’ll do my damnedest to hit one of ‘em.
Truly my favorites are the presentation of a garment “This is really old and we need to add a size (or 3) to it, so can you make this yardage match it … exactly? Um, and we can’t wash it. Ever. The designer likes the aged effect.” This is the stuff that makes my day. I get an enormous kick out of recreating 80 year old patina on silk gazar. Well, it matches in the shop. Make nice with the lighting director and Bob's your uncle.
All of this being a great reason to buy (or dye) enough of the same dye lot of anything. If that’s the sort of thing you want.
There are the tricks played upon you by your own rods and cones. Bright sun light can reduce your sensitivity to color, one eye at a time (check each eye for color perception after waking up from a nap on the beach).
There is also retinal burn out. Staring at a colored pattern then staring at a blank white space should show you a chromatic opposite of the original pattern.
Also contrast or context affect color perception. Here;

same color (#c7d167, check it out on your own monitor) in the middle of each square, flavored by it’s surroundings (you can also play the retina burn out game with this sample).
Maybe I’ve been lucky to run in to the 2 people who see color they way I do. There are reptiles who see in infrared with which they sense heat, a fourth dimension of vision. Imagine the trouble they have getting their shoes to match their party dresses.
“Thus many color matches are conditional.” I shall embroider this on sets of tea towels, each letter a different color. Because … dyeing is a science.