From day to night  using Photoshop layers and quick mask tools

Save the BigBen image to your folder and open it in photoshop. Then go to image/mode/CMYK colour to convert the image to to CMYK this will give you more control over colour channels fig1
fig2 Duplicate the background layer by dragging it onto the new layers icon and rename both layers as per fig2

Nightblue layer

Go to menu/ image/adjustments/hue Saturation and turn the saturation of the Red/Yellow/Magenta channels to -100

click ok 

then Go to menu/ image/adjustments/hue Saturation click Coloize and turn the master channel hue & saturation up till you get a rich blue

 fig3
Select the sky on the colour layer. With the magic wand set the tolerence to about 30. Hold down the shift key to select unselected areas then click the add layer mask icon fig4
fig5

A mask like fig5 should be created

If you press the alt key while clicking on the mask it will open up. You can clean up the black and white areas of the mask using a paint brush.

fig6 fig7

Next, create a new layer and fill with a gradient blue and white fill as per fig6.

this will bebe  background for your sky
fig8 Pull up your mask using alt. Click on the colour layer, select the sky area, then go to select inverse. Select the sky layer and click the add layer mask icon on the sky layer
Create another layer and call it clouds. Go to filters, render clouds . fig9
fig10 Then go to menu/edit/transform /perspective

fig11

change the perspective of the clouds to create a distance effect

fig12

resize the clouds so they will cover your whole sky area

Menu/image/adjustments/brightness and contrast

adjust the contrast of the clouds to make them more interesting

Pull up the layer mask using alt. Click on the colour layer, select the sky area, then go to select inverse. Select the cloud layer and click the add layer mask icon on the cloud layer

Set the fill of the cloud layer to about 70%

 

Next import the moon file and add it to your scene

note:

water reflections and the moon halo will be covered in the lesson