He earlier (ch 3) reminds them to see Jesus as they themselves look in a mirror - reminding them of who they are -. They needed reminding because they were still immature in their revelation of the Good News and the seed was still in danger of being stolen from of the surface. -Remember in Mathew, and the parable of the sower, Jesus says that the soil is our understanding (revelation), and if the truth doesn't go into our understanding (and out of sight from the bird that wants to steal it), then it is vulnerable.
James tells us that the Immature (those with little understanding) are those who look at themselves in a mirror and walk away and forget what they look like. They are the ones who are tossed about by every wind and change of doctrine...
Darkness is only darkness because it has not YET perceived/comprehended that Light shines IN and THROUGH it:(the power of darkness is deception... it hides/covers truth)
Joh 1:4 In him was life; and the life was the light of man.
Joh 1:5 And this Light shineth in the darkness; and the darkness apprehended/comprehended/
Paul makes it clear through his Gospel these people (darkness, temples of idols, unrighteousness) are blind to the accomplished reality. 2 Cor 4:4 - they have blinded minds and have not yet seen the light of the gospel. Col 1:21-They are enemies towards God in their own minds/imaginations and it shows in their actions. They act like Darkness and behave unrighteously because they are still in the dark about who they really are.The power of darkness is deception. The reality, however, is that He has delivered us (ALL) from the domain of darkness and transferred us to the kingdom of his beloved Son (Col 1:13), and has reconcile to himself all things, whether on earth or in heaven, making peace by the blood of his cross (Col 1:20)