The Spot Layer Tool in Preflight can also make masks for data, and in some cases may be the preferred method of spot layer creation. To ensure that mask selection is made in Preflight without selecting those parts of your image that are of the same colour, it is necessary to create a layer in Illustrator to serve as the mask.
Procedure
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Create a box around your image using the Rectangle Tool or other appropriately shaped box tool.
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Make sure this new box is selected and select Fill swatch located at the bottom of the Illustrator toolbar. This will fill the box with colour. By double clicking on this swatch a dialogue box will open allowing for colour changes. Ensure that the chosen colour does not appear anywhere in your image. For this example we have used red (composed of 100% Cyan and 100% Yellow).
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Place this rectangle behind your image data, either underneath or in a new layer below. It is not necessary to choose Overprint attributes for this layer.
Results
The prepared file should look similar to the example below.
Red Mask
