It is important to be consistent in design throughout a publication. This particular example, Snitt, uses the colour purple throughout to keep consistency, and also to keep things simple. The reader realises the identity of the publication throughout and this gives it a certain aura - it shows the publication is a whole work of design and not multiple individual pages.
There is also evidence of a set going on - distinguished by different colours. This shows that the use of colour can be used to separate parts of the set, and the consistent design styles demonstrate the separate publications are part of a whole body of work.