Remove Color From a PDF: Why Green Turns Lighter Gray Than Blue

The obvious way to turn a color into gray is averaging its red, green, and blue values together. That's not what produces a visually accurate result, though — a pure green and a pure blue at full brightness would average out to the exact same gray, even though green genuinely looks brighter to the human eye than blue does.

The Formula: ITU-R BT.601

Gray = 0.299 × Red + 0.587 × Green + 0.114 × Blue

This is a broadcast television standard for converting color video to grayscale, and it weighs each color channel by how much it actually contributes to perceived brightness — green contributes the most (58.7%), red contributes a moderate amount (29.9%), and blue contributes the least (11.4%).

The Difference, Demonstrated

Converting three pure primary colors at full intensity:

• Pure red (255, 0, 0) → gray value 76
• Pure green (0, 255, 0) → gray value 150
• Pure blue (0, 0, 255) → gray value 29

A simple average would give all three the identical value of 85. Instead, pure green converts to a gray nearly twice as bright as a simple average would suggest, while pure blue converts to a gray less than half as bright — matching how these colors actually appear in terms of brightness to human vision, not just their raw numeric intensity.

Why This Matters for a Real Document

A chart or diagram using color to distinguish categories — a green line versus a blue line, say — stays visually distinguishable after grayscale conversion specifically because the formula preserves their different perceived brightness. A naive average would risk two different colors converting to nearly the same shade of gray, making a previously clear chart unreadable once the color is gone.