CMYK HSL Picker

What is it?

Usually, HSL color pickers in your graphic design software work with RGB internally. This makes them less than optimal for working with colors that are to be printed because some hue wheel regions (most notably greens) will get crushed to one nondistinct color.

This picker on the other hand uses cyan, magenta and yellow primaries to generate the hue spectrum. This ensures the colors across the hue range are all representable in print. Also, it is useful when you need RGB colors that will be reasonably well printed (e.g. for software that doesn't support CMYK at all)

How does it work?

The standard formulas for converting HSL to RGB are used, only with primaries swapped to CMY. Then, a formula from pdf.js is used to display them in RGB. I cross-checked RGB colors by printing a few swatches on a color laser printer and they came quite close to what I saw on the screen.

Limitations

• For CMYK values there is no GCR (gray color removal) performed. It is assumed to be done at later stage. If the downstream processing doesn't do GCR, the resultant TIC (total ink coverage) may approach 300% for very dark colors, which may be out of spec for some print processes.
• The displayed RGB colors are a good, but only an approximation. It does not use the whole LUT of an ICC model, only three four-variable quadratic polynomials fitted to it.