What he's getting at is probably along these lines:
I define the "God" I'm looking for as an invisible green humanoid. Because something cannot be invisible and green at the same time, this "God" cannot exist.
It's naturally much harder to do this with Gods that correspond to particular religions, but from time to time it is possible. E.g. a jealous God that will destroy civilization if we don't offer it human sacrifices has not done this, so it can't exist.
Do you mean "which reflects or emits light of these particular wavelengths", or do you mean something about human perception?
Also, similar questions apply wrt "invisible".
If "green" is based on human perception, why would not an entity which, when a person looks in their direction, does not really see them like any shape, but rather, perceives some greenness in that General direction?
Why would this entity not be considered both green and invisible?
Alternative, does "invisible" mean to the naked human eye?
I define the "God" I'm looking for as an invisible green humanoid. Because something cannot be invisible and green at the same time, this "God" cannot exist.
It's naturally much harder to do this with Gods that correspond to particular religions, but from time to time it is possible. E.g. a jealous God that will destroy civilization if we don't offer it human sacrifices has not done this, so it can't exist.