I just read a fascinating article in the
Winter 2011 edition of Plein Air Magazine
about a group of 19th Century Italian painters
called the Macchiaioli.
(Macchia refers to the overall effect
or the value structure of the composition.)
Their goal was to translate the harsh contrasts of the
Tuscan sunlight and shadow
by using a simple patchwork of lights and darks.
They would do small plein air sketches on location
to use as reference for larger works done in the studio.
Roofs in Sunlight by Raffaelo Sernesi ca. 1860 -61
oil on cardboard 5 x 7 inches.
Unlike the Impressionists, these "sketches" were never exhibited.
The first time they were seen was in Florence in 1905
where they caused an immediate sensation.
(Google Images for more examples)
Reading about them inspired to try this.
"Roofs in Sunlight"
10 x 12 oil
"The Macchiaioli sought to summarize their experiences
abstractly through patterns of interactive color-shapes"
That, to me, is the eternal challenge!