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 <WSD>light</WSD> , the middle tone , and the darkest dark - by reducing the forms of my subject to these large patterns . 
Meaning of the word marked with <WSD> tags:  the visual effect of illumination on objects or scenes as created in pictures