If i got the green fire activated as destro warlock, the "Auto/Green" Spell Icons setting doesn't actually work for me. The first spell i cast (Immolate) will be green, but then it starts switching to orange spells.

Found the issue. It will be fixed in the next update.
Thanks for the report.

If you want to manually fix the issue before the update, change those lines :
https://github.com/SimCMinMax/AethysRotation/blob/master/AethysRotation_Warlock/Destruction.lua#L176
if Settings.Commons.SpellType=="Auto" then --auto
TO : if Settings.Destruction.SpellType=="Auto" then --auto

and https://github.com/SimCMinMax/AethysRotation/blob/master/AethysRotation_Warlock/Destruction.lua#L181
elseif Settings.Commons.SpellType=="Green" then --green
TO : elseif Settings.Destruction.SpellType=="Green" then --green