Full-color or 4-color printing is created using what’s called (redundantly) 4-color process. This involves the printing company digitally separating your color files and images into ‘dots’ in each of these 4 colors: Cyan (bright blue), Magenta (bright pinkish), Yellow, and Black (aka “K” in the print world). These dots print as very tiny dots of ink (which you can usually only see using a magnifier), and in various sizes. As you can imagine, the larger the dot, the more of that color is saturated in that area.
Using 4-color process, a very wide array of colors can be achieved using conventional inks on an offset printing press, but not all - specifically colors that are brighter than the base inks themselves (such as florescent shades). However, deep blue skies, attractive flesh tones and really appetizing food can all be achieved using these 4-colors - it’s amazing.
Offset printing also has a significant advantage (over digital printing) in being able to manipulate colors on press to achieve specific results. Flesh tones, for example, have magenta and yellow as their dominant colors (all 4 colors making up the flesh tones, but cyan and black are mostly in the shadows). On press, we can slightly adjust the amount of magenta to give the skin a more healthy look... but too much can make the person look sunburned. And, if the saturation of yellow is too high, the person can look jaundiced. Color balance on print projects with food images is also very important - we can adjust color to literally make food look ‘cold’ or ‘hot’!
Having said that, 4-color digital printing has been advancing steadily in quality over the past decade, now being substituted for traditional offset printing where applicable. For many projects, digital printing is the only cost-effective way to produce the finished pieces, and in those cases, is typically perfectly acceptable in terms of color and image reproduction quality. As you might imagine, however, it does not match the quality of traditional offset printing, especially in blue skies, flesh tones, and food.