Flyer Printing

4. Trapping treatment of spot colors
Since spot color is different from printing 4-color, two spot colors will not overlap to form an intermediate color, intuitively, it is a dirty color effect (of course, there are exceptions, for example, a third color acquired by overlapping and mixing is full, strong and actual, such as a steady dark red acquired by impressing spot red on spot yellow is usually used for book covers), therefore, when defining spot color, overprint is commonly used instead of keepaway, thus, only if there is no other colors beside the spot color, it can carry our trapping treatment to avoid from exposing of white edge. (Since the trapping method and technology are very complicated, we do not discuss for details)

5. Spot colors matching system
When selecting computer color matching system, you shall first understand which are spot color systems and which are CMYK 4-color systems, so that it can be clear if there are colors transformation and distortion when separating output in the system you choose. For example, the common color matching systems are PANTONE、TRUMATCH、FOCOLTONE、TOYO ColorFinder、ANPA-COLOR、RIC Color Guide, etc. Among the above systems, TRUMATCH、FOCOL TONET and ANPA COLORR are based on 4-color printing while others are spot color system (PANTONE covers a wide area, including 4-color and spot color). It means colors of spot color system PANTONE, TOYO Color Finder and RIC Color Guide are much more than 4-color field that can not be presented by 4-color; the CMYK combination values are just approximation. If you choose colors of spot color matching system and output with 4-color, or you refer to the correspondent approximate combination values and use them to replace the spot colors by 4-color, it can lead to distortion; second, you shall understand that when the spot color matching system is carrying out color separation, each of the spot color will be separate independently as a color plate, unless the CMYK 4-color printing has being pre-set.

Similarly, after coloring the colors in 4-color matching system, they will be onto 4 plates according to CMYK matching values when separating output. Second, when the colors are on pictures in a dot matrix environment, it will be automatically transformed into a color mode (such as CMYK 4-color mode) same as the data file. Therefore, in dot matrix drawing software, spot colors shall be specially defined or added (for example, there are two situations when adding or defining spot colors in Photoshop: first, if use spot colors in the whole picture, it has to transfer the picture into double-color mode, and apply spot color in one of the double-color plate; if use spot color in a special part of the picture, it has to establish a channel for spot colors). In a vector drawing environment, if select or define a spot color, it will exist with its property, unless the property is specially changed or transformed into printing color (for example, add or define a spot color in liiusttrator, even the spot color is cited to other liiusttrator data files or open the data file with other vector software, the color property will not change). 
6. Cost of spot color printing
Spot color printing is usually used for printing less than 3 colors; for printing more than 4 colors, it is recommend to use CMYK printing. Because CMYK printing is based on dots overlapping while spot color printing is solid. Although spot colors are usually used in part of picture, its price is estimated twice of the ink price. Besides, if there are already 4 colors in a same page, it means one color more in regard of printing. In case the printing machine has no spare printing unit (machine less than 4 colors), time will be doubled, so the cost will be higher.
In a word, much attention shall be paid to spot color, or it may result in distortion, destroying your ideas and designs, higher cost as well as time waste.