Choosing Inks for Color Printing - Coated vs. Uncoated
Regardless of whether a color is C or U, the ink is made the same. The image to the right shows that PANTONE 293 C and PANTONE 293 U look very different, but are made from the same formula (equal parts of Reflex Blue and Process Blue.) Since coated papers allow the ink to sit on the surface, it remains rich and vibrant. The uncoated sheet allows more ink to be absorbed into the paper. Sometimes the minerals used as pigment to color the inks effect how it will absorbed and also effects the color.
Notice PANTONE 290 C and PANTONE 290 U are closer in color. This color is made mostly from Transparent White (which you'll remember is essentially "clear" and allows more paper to show though the ink.) Since only 3.2% of the mixture is actual pigmented ink, it's less affected by the coated and uncoated paper. As a result, coated and uncoated versions of lighter colors like yellow and light shades of blue, red, or green, will match more closely, while darker shades and colors will look different... sometimes VERY different.
In fact, some designers will go as far as to choose different spot colors for their files, depending on the stock that's used. PANTONE 710 U don't really match PANTONE 710 C very well, but PANTONE 185 U does match fairly well.
The color difference in coated and uncoated stocks is also true for Process Colors, though for slightly different reasons. Process color allows a wider array of colors due to using halftones and blending tints of each process color. Everything is made up of dots; big dots, little dots, but dots nonetheless (if you need a visual, check our previous blog Spot Color vs Process Color.) These dots of varying sizes are more likely to be effected by something called "dot gain." Remember how uncoated stocks are more absorbent, which means they will be more likely to cause the ink dots to swell slightly? This is dot gain. Most everyone knows the Bounty Paper Towel commercials, where the paper towel is used on a small spill and as the towel absorbs it, the spot on the towel spreads out. Ink on uncoated paper does a similar thing, so a halftone dot of magenta that's set for 50%, could swell up to 55% on some stocks and causing the color to shift slightly.