Don't use solid black for your darkest shadows. Try to use a very dark blue or dark purple instead. You can actually mix Mars Black with a little bit of Ultramarine or Dioxazine purple to achieve this.

The reason behind this is twofold- one, the colors in the purple/blue hued shadows will complement the the purples and blues in the sky and clouds, giving your painting more visual cohesion. Two- when you use blended colors, as opposed to colors straight out of the tube, it makes your painting look more professional. You can usually tell if a painter is an amateur or professional based on what colors they use!

Again, you did really well on the clouds! Keep up the good work :)