Thanks guys,
Regarding the Frog's skin painting process :
I did a basecoat with Vallejo 70.830 (German Fieldgrey WWII)
I quickly highlighted with 2/3 Vallejo 70.830 and 1/3 Vallejo 70989 (Sky Grey)
I washed with a 50/50 Citadel Agrax Earthshade / Nuln Oil
Then a Glaze of Citadel Waywatcher Green (applying with different intensity to have different levels of Greenish)
Then some highlight with Casandora Yellow shade from Citadel (again with different intensity)