Given the time of day you noticed this, was it just about the same time the lights came on? When the Odonus niger
becomes freightend or stressed in any way, they will change color which will cause a patchy appearance in their coloring. The same will happen when they become excited as well.
It is also quite normal for their color to diminish quite a bit when the lights go off for the night. It could simpley be you noticed this during a transition of the lights coming on and the fish becoming active.
Keep an eye on it to see later in the day if the colors are remaining the same and if the "pattern" has changed or not.
What other fish are in the tank and was the trigger QT