Water is the most important ingredient when painting with watercolor paints.
It is very important to know its characteristics or you will not be able to properly handle this medium.
Many of the wonderful effects achieved in a great watercolor painting are directly attributable to the amount of water on the paper, in your pallet mixes and on your brush. By keeping an eye on just how wet or otherwise all of the above are you can create magical wet on wet washes, dramatic dry brush edges or sharp wet on dry lines and surfaces – as well as an infinite range in between.
The sky is usually the lightest part of your painting – better too light than too dark.
If you paint your sky to strong a tone then you will have to paint other parts of your painting even stronger which will reduce the transparency of your painting i.e. if it your paint has to go on too thick to retain its correct tonal relationship amongst its various parts.
This is a simple and maybe obvious point but it is still very important.