My natural colour is a fairly boring light to mid brown with a slight gold cast to it. I really just try and intensify it slightly now, having been every colour under the sun...
If you have been dark all your life then you need a deep ish colour for now. The most important thing is what is your natural hue/cast? This dark brown that was natural, did it shine gold or red in the sun? Or did it have an indigo hue to it? Were you a bluey black or a brown so dark you couldn't see any other major tone to it?
If blue/indigo/black/cool tones, you need a cool or ash colour, if warm/red/mahogany, you need a golden or auburn or warm brown.
You say your skin is paleish olive and your eyes hazel, you might be cool. Apparently Olive skin is often cool, you may have cool tones. Or not. The key is whether or not it shone red in the sun in its natural state.
If you are warm/red/gold etc then I wouldn't go darker than a medium golden brown if you are trying to lighten up a bit. But I woudn't go much lighter than that or you might go ginger (depending how much grey you now have). Nice n Easy 117 would be a good colour for your roots. If cool, 117A natural medium ash brown might be good for your roots.
For the body of your hair, it depends a bit what colour the ColourB4 reveals. See my result on my photo - that's a Light Ash Brown, yet it looks well into the medium zone. So if you reveal an auburnish colour or a dark ginger, you are already nearly a medium in terms of colour depth so you don't want to slap a medium colour on top of that, even though you will probably use one on your roots in future. You'd probably slap on a Light with a tone that neutralises the ginger.
So even if your natural colour is quite warm, you may not need to add warmth after ColourB4 simply because of your ginger base. You'd do a 114 Light Ash brown then do your roots in 117 Medium Golden Brown.
I know it sounds complicated and risky but trust me, with strand tests you will be fine and can't go too wrong. If your roots and base are all one uniform colour after ColourB4 then you can just slap on one colour but you may need to adjust what you use on regrowth once that comes through, due to the lack of ginger.