Complex sentence: Picasso's Blue Period (1901--1904), characterized by somber paintings rendered in shades of blue and blue-green, only occasionally warmed by other colours, began either in Spain in early 1901, or in Paris in the second half of the year.
Split into simpler sentences: Picasso's Blue Period (1901--1904) consists of somber paintings rendered in shades of blue and blue-green, only occasionally warmed by other colours. This period's starting point is uncertain; it may have begun in Spain in early 1901, or in Paris in the second half of the year.