1. What is spot color?
In offset printing, a color that is produced by a ready-mixed ink is called spot color. There are several spot color system created. The Pantone Matching System(PMS) created by the Pantone Inc. is the most dominant and most widely used.
2. What is process color?
Process color is also refered to as CMYK color, four color. To have a better understanding of the process color, the term of CMYK color model and halftone needs be explained first.
2.1 CMYK Color Model
In contrast to the RGB color model, CMYK color model is a subtractive color model, used in the color printing. CMYK refers to the inks cyan, magenta, yellow and key(black).
The overprinting of the CMYK color can reproduces other colors. Cyan and yellow produce green, cyan and magenta produce a purplish blue, yellow and magenta produce red.
When we create dots small enough and dense enough on a paper, and see them far away enough, the dots blend together into continuous tone. The is our optical illusion. Therefore, halftone is a technique, using half tone dots to create the continuous tone imagery.
Left: halftone dot pattern with increasing size downwards, Right: how the human eye would see this, when viewed from a sufficient distance
With CMYK printing, halftoning (also called screening) allows for less than full saturation of the primary colors; tiny dots of each primary color are printed in a pattern small enough that human beings perceive a solid color. Magenta printed with a 20% halftone, for example, produces a pink color, because the eye perceives the tiny magenta dots on the large white paper as lighter and less saturated than the color of pure magenta ink.
Without halftoning, the three primary process colors could be printed only as solid blocks of color, and therefore could produce only seven colors: the three primaries themselves, plus three secondary colors produced by layering two of the primaries: cyan and yellow produce green, cyan and magenta produce a purplish blue, yellow and magenta produce red (these subtractive secondary colors correspond roughly to the additive primary colors) plus layering all three of them resulting in black. With halftoning, a full continuous range of colors can be produced.
As for now, you may have known what process color is. If no, check the animation below.
3.How to tell if a color is spot color or process color?
A 15x magnifying glass, or much powerful, is perfect to tell the difference between a spot color and a process color.