Youhou !
I get it !
Here’s where I am now :
It’s much better, no ?
I sleep on this and reread your answer this morning, I see this :
My source light was set to an intensity of 2.
By setting it to a much higher value (30) and with a very low extinction in my material, I manage to get the effect I want 
I use the “overide Light Color” too… It can be really cool or problematic, depending of angle of camera toward the light source.
Maybe I will change my mind on this ^^
I also finally understand that the albedo slot in the material can receive a color and is not “scalar only”.
that’s how I manage to give my myst the bluish color I wanted.
I use a very small amount of emissive in the material to compensate the dark zones where my myst is too dense.
Here’s my “final” material :
I will soon put every settings of it in a paramCollection to drive them in runtime
Add some contrast by postProcess…
Thank again, I was really stuck and I’m on the road again ^^