Unless the formula is proven wrong, to know how much you need to be dark green. (It's the lazy way / easiest) It was tested on lvl 1, 2, 3 but i seriously don't have the time to do this for every freaking lvl.
Formula : (Portail lvl x 1090) + 27 250 = Dark Green Lowest value.
Ex :
So to beat the Portail lvl 1, it would be
(1 x 1090) + 27 250 = 28 340
Lvl 50 it would be ...
(50 x 1090) + 27 250 = 81 750
Lvl 136 it would be ...
(136 x 1090) + 27 250 = 175 490
*****
If you like the hard way or want to know all color ...
The original formula, is : (Portail lvl x 600) + 15000 = Olv (Orange Lowest Value)
Olv x 1.5 = Yellow lowest value
Olv x 1.65 = Light Green lowest value
Olv x (109/60) = Dark Green lowest value
(109/60) = 1.816666666666667 (So it's shorter to make 109/60 instead and you don't have to remember how much 6 you need to write
In short, each lvl ask 1 090 more than the previous lvl to be dark green.