I have two greens on my palette, they are viridian and chromium oxide. I will discuss one or two more besides.
Viridian hydrated chromium oxide is a cool, permanent green first introduced in France in 1859. It is non poisonous, that is important because it replaced an earlier color called emerald green.
Emerald green was copper aceto-arsenate and was so marvelously poisonous that under the name Paris green it was used to poison rats in the Paris sewers, hence the name. It was used as an insecticide until the invention of the far less toxic DDT. Many artist were poisoned by emerald green and has been long since removed from artists palettes.
Viridian is a deep green that is transparent without any olive or redness to it, so it looks very "clean". In recent years it has become expensive and the quality seems to have dropped. Viridian has a tendency to become gritty on the palette before it is used up, so learn how much you are likely to use in a day and put out only that much.
Virifdian is great for painting foliage and for seascape. It also makes a cerulean-like color when mixed with a great deal of white. That is handy in skies. Mixed with cadmium yellow it makes the color of grass in sunlight nicely, but one must be careful lest every color in a painting is made of viridian. Because of its cost, many manufacturers now make viridian hues which are pthalocyanine and handle nothing like the real thing. They are to be avoided. If you can't afford real, mix your greens from your blues and include a pthalocyanine blue if you want that level of pigmenting strength. I have tried to do this substitution for years and always end up returning to viridian. I think it is a very valuable color on the landscapists palette. I buy mine from RGH, and if I buy it by the quart it is affordable. A quart will easily last me a year.
My other green is chromium oxide. That is an opaque and warm color with high covering power. It is dull like an earth color and is very useful in painting greens in a landscape that are not too assertive. I use as much as I can in the summer, if it is less true than the intense greens of high summer it is also less assertive. I believe that Willard Metcalf relied heavily on chromium oxide green. It is a moderately priced color and is premanent.
Pthalo green is an intense green that I think can be easily mixed from pthalocyanine blue itself. It is so powerful that it is difficult to control and can overwhelm the inexperienced artist.
COLOR IS A WONDERFUL SERVANT BUT A DREADFUL MASTER.
I advise against the use of pthalocynanine for the inexperienced painter. However many fine landscape painters have relied on it. In New England Emile Gruppe is the prime example as he was an exponent of the color.
Sap green was a lovely organic looking green that I used a lot in the 70's it was made from buckthorn berries, had a "whiskey colored undertone and was impermanent. Today sap green is not a pigment but is mixed from pthalocyanine and whatever else to make a color that resembles the old pigment not at all. I feel the same way about this as I do about any other pthalo based color, just buy the blue and mix your own notes.When you see a tube labeled permanent green it too will be pthalocyanine, but could be any shade of green.
Terra verte is as its name would indicate, an earth color. It is rarely used today and is more likely to be found on a portrait painters palette.
Olive green is a mixture, again often containing pthalocyanine, that has a brown tone usually obtained by the addition of a red earth color.
Soylant green is people.