Covering a Pastel Picture in White Before Adding Colour

We recently had this great question come in from Irene:"I have been told that the best way to approach a pastel painting is to cover the subject matter in white before adding colour. Is this so and if so why?Your comments would be much appreciated."The Pastel Pencil medium is different in make up to the traditional soft pastels; therefore they need a very different approach.The pastel pencils are more translucent than the stick pastels and as a general rule used from light to dark similar to the watercolour approach. Whereas you could (although I would not do this) use white or a similar light colour to use as a base colour and the stronger colours of the soft pastels could be used on top - even the dark tones.If you tried this with a blanket covering of white as a base for pastel pencils then tried to use strong dark colours on top of this, the base colours would look washed out and the results would be unacceptable.I do often use white and other lighter colours as a base for mid tone shades such as white for a light blue sky or a light grey base for water, backgrounds etc.When I want to apply dark colours I would choose a suitable medium tone of base colour, the dark colours can then be added to this base colour.I hope that helps to answer the question and differences between soft pastels and the way we work with pastel pencils.