How To Preserve Spot Colors in Photoshop (version 6 and higher)

Now with your color picker, click on the star. Choose the Brush Tool. Make sure it's a big brush and it's a solid brush (no feathered edges). With your selection still active, paint around the edges of the star. You are basically painting that extra pixel with the spot color of the star. Once you are done, make the CMYK channels visible, together with the spot channel. All the colors should touch each other perfectly.

What you have just done is called trapping.