I have a dark green screen and when I try to adjust the color of the key it seems to have no effect, have tried on other colors as well and the key seems to have no bearing.

Here I have set my key to white, and held up a white cloth and a green one, and it is still looking for green instead of white.

Greens and Blues are probably the easiest colours to get a good Key on (there's a reason why "greenscreen" has become the industry standard).
When using another colour or using the colour picker in vMix, you will likely have to play around with the various sliders in the Colour Key Window (Red/Green/Blue) to effectively remove all the colours in the spectrum that might be present in the colour you chose, which may not yield the results you want in other parts of the image. Whites and Blacks can be particularly problematic (they are not really "pure" colours in and of themselves, but rather more the brightness and darkness of colours).
It is also important to keep in mind that what a camera sensor and/or GPU and its screen sees is significantly different from what the human eye sees.
Many have previously indicated that the vMix Keyer is not the most sophisticated of Keyers, but with good lighting and a decent "greenscreen", you can get pretty good results.
If you Stray from the "norm", you are bound to have mixed results.


vMixGreg