Sentence:  Mix ultramarine blue and alizarin crimson in your palette until you get a rich purple, then paint a <WSD>swatch</WSD> on dry watercolor paper [...]. Next paint a <WSD>swatch</WSD> of ultramarine blue on dry watercolor paper. While this is still wet, add alizarin crimson to the lower part of the blue wash, and watch the colors connect and blend [...]. Compare the two <WSD>swatches</WSD>. 
Meaning of the word marked with <WSD> tags:  A clump or portion of something.