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If you are using a spot color, select your color from the Color Mode menu.
Open your Swatches panel and select New Color Swatch. Select either Process or Spot as your color type. Select a mode for defining the color. Drag the color sliders to adjust the color values or enter specific numeric values in the boxes located by the color sliders. Click Add to add your swatch and create a new one or click OK to add your swatch and exit the window.