Fox McCloud's eyes are magical, and not because they're so pretty.*

In English, there are eleven basic color terms

— black, blue, brown, gray, green, orange, pink, purple, red, white and yellow. These colors are fairly consistent, each with culturally canonical hues, by which similar hues are usually associated — for instance, scarlet is considered a type of red, gold is considered a type of yellow, etc.

In the Sinosphere

— the regions that either speak one of the Chinese languages (such as China, Singapore, Taiwan, etc.), or have languages that incorporate massive amounts of Chinese-derived extended vocabulary and have historically made widespread use of Chinese written characters (such as Japan, Korea and Vietnam), these regions traditionally have the same word for both blue and green

, indicated with the Chinese character 靑 (or its alternate glyph 青).* Most natural and traditional uses of both blue and green are represented by this word, including the color of the sea, the color of forests, etc. In more recent centuries, there has arisen a greater need to distinguish the concepts that English-speakers would understand as blue and green. The newer compound Chinese character 綠 came to use in Chinese, Japanese and Korean to specifically mean green as opposed to blue.*

However, even today, these two terms are not universally distinguished as would be understood in English. For example, forests are still 靑 (blue). Green eyes are also confusingly 靑 — they were known to traditional Chinese civilization because there were ethnic groups on the periphery of their civilization (such as the Tocharian and Turkic peoples) who often had green eyes. And even green traffic lights are 靑. But not all "natural" green things are 靑 and not all "modern" green things are 綠 — for instance, gemstones such as jade and emeralds are 綠 (green). Perhaps most confusingly, even though forests and grass are 靑 (blue), verdant flora is 綠 (green).

And now where this ambiguity becomes a trope. It is most easily noticed in Japanese entertainment that is in full color (such as anime and Video Games), particularly with character eyes. (This is mostly irrelevant in manga, which is usually in black and white.) A character's canonical physical appearance may have 靑 eyes, but may be inconsistently portrayed as having blue or green eyes within the same series, or sometimes within the same work. Since the vast majority of Japanese people have brown eyes, this mostly affects Caucasian characters, or characters that are Kemono (Petting Zoo People) — brown eyes in Japan are a predominantly human trait, and non-human animals in and near Japan commonly have other eye colors including blue. Understandably, many viewers even in the West may not even notice this blue/green inconsistency, as it is common to overlook other people's eye color.

In Japan, this may occasionally be a People Sit In Chairs for obvious reasons stated above, where the difference between blue and green is not always considered significant — in the more distant past, this would have almost certainly been true. But this becomes far more noticeable to foreign consumers of Japanese entertainment, particularly to those people who are detail-oriented. Where this becomes more blatantly obvious, it can be considered a Language Trope. And, as mentioned earlier, the difference between green and blue is now well-known in Japanese culture, but it is the indigenous terminology that can be ambiguous.

Note also that, the world being a diverse place, the Sinosphere is not the only place where languages often muddle the distinction between green and blue. This has also been observed in the modern Celtic languages (Irish, Welsh, etc.), where there is not only some muddling between green and blue, but also between green and gray. Similarly, older Italians lump orange in with red.

Not to be confused with the (rather confusing) Dub Name Changes for the characters named Green and Blue in various Pokémon media.

Read Guy Deutscher’s Through the Language Glass for an in-depth explanation of this trope and its equivalents in other countries.

Examples

In Dragon Ball, the Super Saiyans eye color is usually greenish in the anime, but from time to time they appear blue in some Toriyama illustrations, as well as in some promotional media and certain isolated anime episodes.
In Tokkei Winspector, the heroes are meant to reflect the traffic lights. With that said, Walter was bluish than greenish.
- Yuno's drunken rant in Hidamari Sketch, currently Non-indicative Name's page quote, is also about green traffic lights.
Similar to Winspector, Tokusou Sentai Dekaranger/Power Rangers S.P.D. has the finishing move of the main robot using traffic lights... with the colors being red, yellow and bluish green.
- "Green" lights are tinted blue, so that red-green colorblind people can distinguish between a green light and a red light.
In the early days of Super Mario Bros., the color of Luigi's clothes was inconsistently portrayed as blue or green. It took a little while before the vivid green color became firmly established.
Fox McCloud from Star Fox is one of the better documented examples of this trope. In the 1993 comic, his eyes were green in the early pages, then blue through the rest of the comic. They remained blue in Star Fox 2. Star Fox 64 had a particular Art Shift that did not show eye color at all, but Farewell, Beloved Falco and Star Fox Adventures firmly established him with emerald green eyes. But this began to slip again in Star Fox Assault, where most of the official art showed him with green eyes, but at least one picture not only showed him with blue eyes, but the blue faded to green within the same irises. They're green again in Super Smash Bros. Brawl.
Torahiko Ōshima from Morenatsu is usually drawn with blue eyes, but some of the artwork shows him with green eyes.
In The World of Ginger Fox, Ginger's eyes are sometimes blue and sometimes green. The cover art shows her with an eye color partway between green and blue.
In Metal Gear Solid 3, Big Boss' eyes are described as blue in dialogue, but they appear green. Snake's eyes were also dark green in Metal Gear Solid 2, but described in his bio in Metal Gear Solid 4 as blue and appear clearly blue in that game.
Several Pokémon are listed as "green" in the Pokedex, when most Westerners would consider them blue: specifically, Bronzor, Bronzong, Golett, and Golurk are all listed as "green".
- There are several orange pokemon as well. But the pokedex ends up listing them as either red or brown.
Qīng Lóng/Seiryuu of The Four Gods is called the "Azure Dragon", despite his element being wood, so one would think it would be colored green.
- Somewhat justified in that wood is associated with air in Wu Xing (contrary to the japanese "translations" listing Seiryuu as earth and Byakko as air), so you can eithe go along with Wind is Green or simply see the Azure Dragon as a manifestation of the sky.
In Japan, "go" traffic lights are green, but art of traffic lights is blue. This shows up in an early episode of Transformers Cybertron, where we see an actual traffic light (well, it's a robot in disguise, but still), and a slideshow presentation of a traffic light, and they're different colors.
- Related: The three forms of Kamen Rider Accel are supposed to be based off of the three colors in a traffic light. These forms are colored red, yellow and, you guessed it, blue.
The Odyssey and The Illiad never mention color blue. It might be slightly odd, given all the sea-faring in the Odyssey, but that was caused by the ancient Greek language inverting the trope - there was no word for blue, only green. Luckily for modern readers, sea is not referred to as green anywhere in the books, avoiding confusion.
Harry Potter is described in the books as having Green Eyes - but in the films they're blue, as Daniel Radcliffe couldn't stand green lenses. (luckily, his on-screen mother - from which Harry inherited the eye color - also has them blue)
Amy Lee of Evanescence did an interview on Tokyo FM, and was complimented on her green eyes. This is where it gets complicated. It's been said that she has green eyes naturally, and wore blue contacts around the time of the first album. This interview was near the time of the second album, but in her childhood photos she had blue eyes. It gets really complicated, because in the Japanese translation, the DJ used the English loanword グリーン, or green.
Miku Hatsune's thematic color tends to fluctuate between any given shade of green or blue, depending on the artist.