Factors Causing Color Variation in Spot Color Printing

In the printing process, numerous factors contribute to color deviation in spot color production. The following is a detailed analysis of these factors:

1. The Influence of Paper Substrate

The impact of paper on ink layer color is primarily observed in three areas:

• Paper Whiteness: Variations in whiteness (or paper tint) directly affect the visual appearance of the ink layer. For high-lightness colors ($L^* > 70$), differences in whiteness can necessitate significant adjustments in the black ink component of the spot color formula.
• Absorbency: Identical inks will exhibit different gloss levels on substrates with varying absorbency. The fiber structure and coating thickness determine how much vehicle is absorbed. Uncoated paper often results in a duller, more "drifted" color compared to coated paper, particularly with cyan and magenta hues.
• Smoothness and Gloss: These properties dictate light reflection. On smooth, high-gloss paper, light undergoes specular reflection, allowing the observer to see the pure color of the ink. On rough paper, diffuse reflection occurs, mixing white light with the colored light, which lowers saturation and increases lightness.

2. Surface Finishing Effects

Surface treatments like lamination (gloss/matte) and varnishing (aqueous/UV) cause both physical and chemical color shifts:

• Physical: Gloss finishes increase color density and saturation, while matte finishes decrease measured density due to increased light scattering.
• Chemical: Organic solvents in adhesives and UV primers can react with ink pigments, causing discoloration or fading.

3. Impact of Extenders (Reducing Agents)

Extenders are colorless, transparent pastes used to dilute color intensity. However, adding extenders alters the distribution of pigment particles. Since the shear force and pressure of lab proofers (e.g., IGT or Ink Proofer) differ from actual presses, a "systemic color difference" often arises between the color card and the final printed product.

4. Dry-back Effect (Density Difference)

Freshly printed ink (wet state) always appears more vibrant and has a higher density than dried ink. This is known as the Dry-back phenomenon. As the ink levels and dries, reflection shifts from specular to diffuse, making the color appear duller.

• Solution: Using a densitometer equipped with a polarizing filter can eliminate surface specular reflection, providing wet readings that are much closer to the final dry density.

5. Systemic Variables

• Emulsification: Lab proofing is a "dry" process, whereas offset printing is "wet." The introduction of fountain solution causes ink emulsification, which alters pigment distribution and results in a less vibrant, "greyer" appearance.
• Production Stability: Other variables including ink film thickness, weighing accuracy, press speed, water-to-ink balance, and the mechanical condition of the inking zones all play a role in color consistency.