Hi Jerome
it is a blending-combination of different layers (monochrome, textures), luck and a bit of skills ... based on a wellbalanced overexposed file.
It is like Gerhard Richter, the great german painter said: " ... You start on it, you give, you take - without really knowing when it is finished. But then you know: it is finished."
Honestly, my advantage on Gerhard Richter is, that I roughly know, what colors I'm aiming for ...