Sentence:  First I make preliminary watercolor sketches in quarter scale ( approximately * * f inches ) in which I pay particular attention to the design principles of three simple values - the lightest light , the middle <WSD>tone</WSD> , and the darkest dark - by reducing the forms of my subject to these large patterns . 
Meaning of the word marked with <WSD> tags:  a quality of a given color that differs slightly from another color