In past (and now too) many times I had struggled to get colors "as seen" (whatever that means). It is often very difficult to reproduce the colors seen by our eyes/mind on a screen (harder on paper). Color cast is one of the many problems. At times cast is so subtle that you can't even make out it exists. Grey cards solve this issue but introduces a new one. At times subjects will have natural color cast (for example sunset/sunrise/warm images). Use of grey card will eliminate them too! One of the other challenging issue is illusion created in the brain as explained somewhere in this nice video on Goethe's work. It is a long watch (~50 minutes) about "theory of color" in general, narrated/presented artistically. I think my time watching this was worth.