This is related specifically to Saintpaulia. I've noticed that sometimes the veins are a lighter green than the leaves. I have a few plants right now that are doing this. I've seen photographs of other varieties where one person's plant shows this, yet another person's plant is the same color green in leaves and veins. Here is a picture of it happening in another person's plant. Here is an example of it in the same cultivar - one with it and one without it. What does it indicate when the veins are a lighter color that the leaf? Does this state mean the plant is somehow getting something less or more than optimal, such as light or nutrients or that soil pH or cellular pH is higher or lower than it should be?