I think you are fine. Sometimes you will find variations in colour between shrooms ie some darker and some lighter. So long as the shrooms are growing well and you have no odours or obvious deformities or coloured patches you are fine.
It could just be that you have a few that are starting to abort. Keep an eye on them and if the cap turns completely dark remove them as they are probably aborting------------------
Be sure to check out www.3mshrooms.homestead.com for your spore needs