Beginner Gardening: yellow leaves and water, 1 by michaelangelo

michaelangelo wrote:
I am almost always at a loss about yellow leaves signaling too little or too much water. The accompanying pic of a cucumber and Impatiens (both in containers) is what sometimes happens (although I have other cucs and Impatiens in the same kind of containers that are very green.

When I find a plant like this, is it best to let it dry more or add water? And when in doubt, what's the most ideal thing to do.