About HexToCMYK Prepress Lab

Paper-aware proofing, not textbook math

The blue that came back purple

#0044FF on coated stock
Screen sRGB #0044FF
Coated simulation Coated CMYK
Build CMYK TAC
Textbook formula C0 M0 Y0 K0 0%
Coated, with the blue correction C0 M0 Y0 K0 0%
Uncoated, tighter cap C0 M0 Y0 K0 0%

A purple blue started this lab

I built HexToCMYK Prepress Lab after a royal blue logo came back purple from a short-run printer. The converter I trusted used one textbook line: K = 1 - max(R, G, B). That line is fine in a classroom. It is wrong on paper.

Real cyan ink is not a perfect filter, so a bright blue picks up too much magenta and dies in the shadows. I wanted one page that shows the screen color beside a coated sheet and an uncoated sheet before anyone exports a PDF.

Screen vs paper The hex you typed, next to a simulation of the same color in process ink.
Ink cap Coated work stays near 300 percent total ink. Uncoated work stays near 240 percent.
A file you can place A binary .ASE swatch for Illustrator, InDesign, or Photoshop, built in the browser.

What the proof is for

The lab is a prepress check, not a brand studio. You paste a HEX or move the ink sliders, read total ink against 300 percent or 240 percent, and see whether a reference chip is closer than process color.

Then you download the swatch. I keep the math in the browser. There is no account and no upload step for a single color. The lab is at 1209 Orange Street, Wilmington, DE 19801, USA. Write to support@hextocmyk.com.

What I will not pretend

A screen simulation is not a contract proof. Paper, ink, and the shop RIP still have the last word. Pantone, RAL, and Adobe names on these pages are references so you can talk to a printer. They are not a licensed fan deck.

If a number on a job looks wrong, use the contact page and send the HEX, the stock, and what the press did. That is how the coated and uncoated curves get tighter.