I am chiefly painting landscapes here in Dorset where I live but have difficulty in obtaining the right shade and intensity when mixing colours. Have you any helpful artistic rules of thumb?

Getting the right shade when mixing colours only comes with practice, but there is one absolutely golden rule and that is to put the predominant colour into your palette first and then mix the other colours into it. To get more intensity with water colours, use more pigment and less water.

Hello Ammar.
I noticed you have posted quite a few questions on colour, shading, mixing colours etc. and I want to recommend 2 books that I purchased which I use constantly and I have found a great help to me - and both are quite cheap!
They are ;
"The Complete Artists Problem Solver" by Trudy Friend.
She gives examples on the first page of what has gone wrong and on the next page, what you do to correct it. By showing clearly what the problem is and how you fix it it tends to stick in your mind more than a simple how-to-do book.
"600 WatercolourMixes" by Sharon Finmark.
It takes the 12 basic colours that you find in practically all ready palleted sets you buy, and explores all the mixes you can obtain from them, giving highlights and shadows to create depth and also it gives examples of the colours being used in paintings and how they look.
Both can be bought from SAA and are normally in one of the offers section.
Hope this helps as both of these books I seem to often refer to when I get stuck and need an answer quickly. Happy painting.
Dawn.