Well first off, the sun doesn't need to be down exactly, just not direct sunlight. On a cloudy day, the sky still gives off a good amount of ambient and healthy light(around 2,400 foot candles). The reason is to not burn the plants like redtails said. And giving them just water isn't exactly foliar feeding, and also, unless the plants are completely dry they shouldn't need more water. The discoloration might be due to the over-watering. When a plants roots are wet, they cannot take in nutrients like when they are dry. Think about it like this, when you are eating, you can't take a bite and a drink at the same time, needs to be a separation. When the roots aren't oxygenated enough they start to just adsorb the nutrients in the leaves. Let them dry out and see what happens. The worst thing people do to their first plants is baby them.

-C