According to the gemological definition of emerald green, a dark green qualifies.
Emerald green is defined as a range of green colors, albeit a relatively narrow one with color broken down into three parts: hue, tone, and saturation. Hue is what is commonly thought of as color, tone is how light or dark said color is, and saturation refers to whether the color is a greenish gray, or grayish green. If a piece of green colored beryl qualifies as an emerald, it must be slightly yellowish green to bluish green, and cannot be too light.
Confusing as it sounds, green beryl and emeralds are considered separate varieties of beryl despite both being green colored beryl.
E1432 | medium | play
Emerald ID: E1432
Weight: 1.20 Carats
As shown on the scale here, dark green definitely qualifies as emerald. However, tones that are very light or very dark mess with our perception of color. If the tone gets too light, there is no visible color. If the tone is too dark the color is black.
The further away from a medium tone the green is, the less expensive the emerald will be. Any amount of yellow is harshly judged as well, and is reflected in the price.
As for the saturation, most dealers do not bother stocking anything other than vividly colored gems. If the beryl is a dark green, then it definitely qualifies as emerald color.
The main downside to the dark green color is despite the fact it guarantees the beryl is an emerald, it is not the ideal color for emeralds. The ideal color of emeralds is a slightly bluish green with a medium tone, and vivid saturation. Imagining a better color is impossible. Furthermore, this is the color that commands top dollar amongst emeralds.
Do bear in mind that there are three other important factors influencing the price of an emerald. In addition to color, there is clarity, cut, and carat weight, which makes up the 4Cs of gem quality. Additional factors like international trade conditions, legal mining certification, and treatments are important to price too.
E946 | medium | play
Emerald ID: E946
Weight: 1.50 Carats