Most of the ten G. zebrinus I've got can be very dark.
The 3 biggest ones (around 6 cm TL) have somewhat fainter markings that almost completely disappear when they have a territorial dispute.
But they can also be lightbrown, darkgreen, darkbrown etc.. It depends on the surface they're on.
I've got only one that's a bit different; It's always chocolate brown and has different markings in the caudal fin.
Some pics of the biggest G. zebrinus (6.2 cm TL,5.2 SL)
It's the same as the right one in the first picture of this thread.
And the chocolate brown one.
Again, and a zebrinus with a 'normal' caudal on the right.
The chocolate brown one has only some faint blue at the tip of the caudal.
How many liters have your tanks?
Hillstreamtank 1 : 100 x 30 x 40 cm
Hillstreamtank 2 : 100 x 40 x 50 cm