This is not only a challenge for those who write about colors in their own language, but it makes researching the issue difficult as well. For at some point, yarok does mean yarok, and kachol does mean kachol. But it becomes very hard to determine when that change happens. We've seen the ambiguity with some of the earlier colors, and we'll see it here with yarok as well.
Yarok appears only once in the Tanach - Iyov 39:8, but it is not clear from there exactly what color is being mentioned. A different form of the color - ירקרק yerakrak - appears three times: twice in Vayikra (13:49, 14:37), talking about tzaraat (like tzahov) and in Tehillim 68:14. Let's look at the latter.
The verse mentions בִּירַקְרַק חָרוּץ which we'll translate (for now) as "greenish gold". But the question here is really: is the gold yerakrak (and therefore different than common gold), or is the yerakrak gold (and is all yarok gold)? There is much evidence that yarok actually meant a goldish, yellow color. In Ugaritic yrq meant gold, and in Southern Arabic warq ורק still does.
We also have Rabbinic sources that connect the two. Tosfot (Sukkah 31b s.v. HaYarok) lists a number of such sources: Hullin 47b says yarok is like the yolk of an egg, and Tosefta Negaim 1:3 says it is the color of wax. Tosfot also mentions a very interesting Midrash, on Bereshit 14:14, where it says that Avraham וַיָּרֶק אֶת-חֲנִיכָיו - literally, he sent out his men. But Bereshit Rabba gives a number of explanations, all with the idea that וירק means "he made them shine (like gold)." (See Torah Shleimah Bereshit 14, #67 for a full explanation of the Midrash.)
(As far as the meaning of yerakrak vs yarok, it would seem that Ibn Ezra could be in trouble once again. The Sifra explains yerakrak, as ירוק שבירוקין - the most, strongest yarok. Ibn Ezra writes that the doubling indicates a weakening, so yerakrak would mean a pale yarok. In this case, Modern Hebrew seems to have taken the side of Ibn Ezra, for words with the last two letters of the root repeating - אדמדם, ירקרק - mean a less full version of that color.)
However, there are also Rabbinic sources that identify yarok with green - the above Tosfot mentions them. The Tosefta in Negaim writes that Sumchus said that yerakrak was like "the wing of a peacock." I had thought that was a sign of yarok meaning green, although this site takes an approach I had not yet seen (original Hebrew here):
What the sages called "green" is today called "tzivoni," meaning "colored." For example, the RAMA wrote, "What is called 'blue' is included in the category of green" [Yoreh Dei'ah 188:1]. The Tosefta compares the strongest green and the strongest red, asking: "What is the greenest of the green?" The answer given by Sumchus was, "like the tail of a peacock." In fact, a peacock has 365 different colors, with all possible colors, except red (Tanchuma, Tazriya) ... In principle, "yarok" is any color that is not red, and this led the Maharam of Rotenberg to write in response to a question, "All the colors blue, yellow, and green are included in the color 'yarok', green."
In any case, today yarok means only green. Perhaps it was through the influence of the words ירק and ירקות - meaning vegetables, or in English, "greens". Klein says that yerek derives from yarok, and many plants and vegetables are green, particularly the leaves. In Arabic, the related waraq means "leaf".
According to Steinberg, the etymological development should be reversed. He says the root ירק means "to empty out, to force out" and is related to the word reik ריק meaning "empty". From here the word ירק meaning "to spit" derives. A plant is called a yerek because it "comes out" from the ground. For sense development, he offers the German sprietzen - "to sprout" and spritzen - "to spray".
Rosenthal gives five meanings of yerukim in Israeli slang: a) environmentalists, b) soldiers from field units (due to their uniforms, as compared to Air Force and Navy), c) Border Police, d) dollars, and e) fans of Maccabi Haifa.