The tube color is called the mass tone (or masstone), and the color when thinned down is called the undertone.
For example, what inspired the color treatment on this sketch was the strange behavior of Azo green watercolor. It has a dull olive-green mass tone but the undertone is a rather strong yellow.
The time lapse from Instagram shows the sequence.
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Check out some of our previous challenges:
Paint a Cell Tower Challenge Results
Dead Vehicle Challenge Results
Farmers would pollard willows to produce flexible rods; they'd cut back beeches to make charcoal; and they'd pollard hazels to produce nuts.
It was practiced in Europe and Asia, and among the indigenous populations of the new world.
Pollarding was commonly practiced for tens of thousands of years until it fell mostly out of favor in the industrial revolution.
Vincent van Gogh
By allowing a tree trunk to grow up to the height of a standing person before it bears tender shoots, the shoots and leaves are above the browse line of deer, sheep and other animals.
Vincent van Gogh
The small branches can be cut back again to the same point after two, four, eight, or ten years, to be used for animal feed, firewood, tool handles, or building materials.
Sir George Clausen, The Return from the Fields
Many of us have assumed that this practice disfigures trees, but it's quite the opposite, providing reliable raw materials for people, and a healthy renewing cycle for the trees. A pollarded tree can potentially live for a thousand years.
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According to author and arborist William Bryant Logan, it's not only healthy for the trees, but it expands the environment for a variety of other plants and animals.
Pollarded Willows, 1887 by William Fraser Garden
Pollarding is still practiced in Europe, but it's less common in the USA.
The place you tend to find it these days is in cities, where trees receive more attentive care than they do in the untended forests.
Ivan Shishkin, Coppice (Noon) 1872
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Previous post on William Fraser Garden
"Garbutt would perch on the edge of the table, more like a bird than a draftsman (he never seemed to sit in a chair), and start explaining, and while he talked his pencil would start making a thin line that seemed to meander aimlessly across the paper. We would turn our heads first one way. then the other, trying to see what he was drawing, but the lines resembled a tangled cobweb as much as anything else. Then, suddenly, we saw a deer in the precise phase of the movement we had described; only Garbutt was drawing it upside down so it faced us."
"While we were blinking and trying to absorb that combination of rendition and explanation, he would continue: "Now with a camel, he'll put this leg out first and keep his head down. ..." When he had finished drawing a camel getting up, he would go on to the buffalo, just so we would have a thorough understanding of what was unique about the deer in this particular action."Books:
More in my book Adolph Menzel: Drawings and Paintings 
As far back as Leonardo da Vinci, artists have propelled our understanding of anatomy.
One artist named Santiago Ramón y Cajal (1852-1934) contributed to our conception of the neurons of the brain with his exquisite drawings.
Before his time, anatomists believed that neurons were like a network of connected pipes and tubules that conveyed liquids throughout the brain.
But Ramón y Cajal believed neurons were separate, individual structures and that he could see tiny gaps between them that came to be known as synapses. Together those two insights are known as the "neuron doctrine," and that understanding is fundamental to neuroscience.
Ramón y Cajal shared the Nobel Prize for his discovery with Camillo Golgi, who developed the special staining method that allowed the neurons to be individually visible.
But even Golgi didn't agree that synapses existed, for they were so tiny that they were almost impossible to see through the microscopes of the time.
Learn more:
Wikipedia on: Santiago Ramón y Cajal
Quanta Magazine: Why the First Drawings of Neurons Were Defaced
BBC Science Focus podcast: Your Brain Chemistry and You
Melanie and Kalyptra, a Dryosaurus, take Arthur Denison on a tour of the plant world, from Dinotopia: A Land Apart from Time.
Lutheran church, Ancram, NY, pencil and gray wash, 9 x 12 in.
The gray wash is diluted ink that I carried around in a plastic jar. It works nicely for a quick shadow value, but it's light enough to allow the soft pencil lines to stand out.
"But he also had a very lively imagination and did some amazing skilled drawings - both for fairytales and for the Snorri Sturluson saga of the kings." 
Artists also include Arthur Rackham, William-Adolphe Bouguereau, Gustave Doré, NC Wyeth, Herbert Draper, Howard Pyle, Maxfield Parrish, Frank Frazetta, Winsor McCay, Jessie Willcox Smith, Joseph Clement Coll, Willy Pogany, J. Allen St. John, Dean Cornwell, Virgil Finlay, Hal Foster, and many more.
