With most good colour drawings a light source is always needed. From the right to left, from behind and front, it does not matter where the light source is coming from as long as your shading, of an object/image, corresponds with it. Shading is something that does not come naturally but with practice. As you keep at it you get better and better, so donít be disheartened if at first it does not work.
1. Create an outline of what you would like to draw (either in the guide layer or the canvas and preferably in a dark or light grey), letís just start with a mountain with a tree on it. It does not matter how sketchy your outline looks, because you will be going over it again. Be wary to only use thin lines, so that, if you are in the guild layer, you can see past the lines onto the canvas and, if you are drawing straight onto the canvas, you waste less ink.
Now that we have our guide set up, pick a base colour to start your drawing off and use either the fill tool or the pencil tool to colour in the object.
NOTE: when picking a base colour make sure that you pick a colour that you can put lighter and darker shades of that same colour over it.
A quick colour in of the rest of the objects.
I am going to start shading the tree trunk now. I suggest starting with the lightest colours and working your way to darkest colours. Firstly pick a light source, lets make it come from the top left hand corner. In your guide layer, draw an arrow coming from that corner, so you will not forget where the light is meant to be coming from.
Click on the colour (in your saved colours) that you used for the base colour of the tree trunk. Then make that brown colour lighter so that there is a visible difference between the base colour and the light colour. Also at this point, considering that this is only a landscape drawing, I would turn the guild layer off. People who would be likely to forget where their light source is might like to keep it on or redraw the arrow onto the canvas, but for everyone elseís sake I will re draw the arrow onto the canvas.
Change the transparency of the lighter colour that you have just chosen to make it less visible against the base colour of the tree trunk. Then go over the part of the tree trunk that should be lighter.
Go over the lighter part of the tree, shown in the diagram on the left. You may have to reduce the transperancy to gain a shaded effect.
repeat 6th step as many times as you wish to create the desired effect. you may also choose to use darker colours too.