Printing terms explained: Knockout, Overprint, Four-color Black, and RGB Images
1. Knockout
If blue text is placed on a yellow background, the area where the blue text appears must be left empty (knocked out) on the yellow plate. Likewise, the same area should be removed from the cyan/blue plate. Otherwise, the blue ink will be printed directly over the yellow, causing a color shift — the blue text may appear greenish instead of pure blue.2. Overprint
If black text is placed on a red background, the red plate should not be knocked out where the black text sits. Black ink can effectively cover underlying colors. If the black area is knocked out, even slight misregistration during printing can cause white gaps or halos around the text. Because the contrast between black and white is high, these defects are especially noticeable.3. Rich Black / Four-color Black
This is a common issue. Before output, check whether black text — especially small text — exists only on the black (K) separation and not on the cyan, magenta, and yellow separations. If black text is present across all four CMYK separations (C, M, Y, K), the printed result can look muddy or fuzzy instead of crisp. When RGB graphics are converted to CMYK, black elements often become four-color black. Unless a four-color black is intentionally required for visual effect, set small or body copy text to 100% K only before CTP plate making.4. RGB Images
When RGB images are sent to the RIP, they are typically converted to CMYK automatically. However, this automatic conversion can significantly affect color accuracy and saturation, producing printed results that look dull or less vibrant. For best results, convert images to CMYK in Photoshop (or another color-managed editor) and perform necessary color correction and proofing before outputting for CTP plate making.
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