CMYK and Lab colour documents in Paint by Zaweba
Paint by Zaweba keeps true CMYK and Lab values on every pixel layer, so print files and Lab TIFFs open, edit and save without a trip through RGB.
Some image editors that open CMYK files quietly convert the file to RGB when it opens and back again when it saves. The numbers a printer cares about drift on the way. Paint by Zaweba takes a different approach: choose Image > Mode > CMYK Colour or Lab Colour and each pixel layer keeps its own C, M, Y and K values, or its L, a and b values, at 16 bits each, held beside the RGB the screen needs.
The practical result is that pixels you have not touched keep exactly the values they arrived with. If a designer sends you a CMYK TIFF with a rich black set to particular percentages, and you only retouch a corner of the picture, the rest of the file still has those percentages when you save it.
Why CMYK values matter
Printing presses lay down cyan, magenta, yellow and black ink. A colour on screen can be made from many different mixes of those four inks, and the mix affects how the print looks, how much ink lands on the paper and how quickly it dries. Printers and prepress staff set those mixes deliberately. An RGB round trip throws that intent away and replaces it with whatever the conversion decides.
Because Paint by Zaweba stores the CMYK values on the layer, CMYK TIFFs open and save without that round trip. Separations, the per-ink plates a print shop uses, are made from those stored values too, so what you hand over matches what you edited.
What Lab mode is for
Lab describes colour as lightness (L) plus two colour axes, a for green to red and b for blue to yellow. It is device independent, so it is often used as a neutral middle ground between profiles, and it is handy for edits that should touch brightness without shifting colour or the other way round. Lab TIFFs open and save in Paint by Zaweba without being forced through RGB, which keeps that separation of tone and colour intact.
A common photographic trick is to sharpen only the lightness, so colour edges don't pick up fringes. Working in a Lab document gives you the lightness as its own set of values to think about, rather than something spread across red, green and blue.
A sensible way to work
Most jobs still start in RGB. Cameras and scanners produce RGB, and the screen shows RGB. A good pattern is to edit in RGB at 16-bit, set up your working spaces in Edit > Colour Settings, soft proof against the press profile, and convert to CMYK when the picture is close to done. Then do the final checks and small fixes in CMYK, where the numbers you see are the numbers that will print.
If a file arrives already in CMYK, there is no need to convert it out of CMYK just to touch it. Open it, make the change, save it. The untouched areas stay exactly as the sender made them, which is what a print supplier wants to see.
Where CMYK jobs go wrong
Switching modes back and forth on a finished file is the main one. Each conversion between colour spaces involves choices about out-of-range colours, so do it once, on purpose, near the end. Keep an RGB master in .ptx if you might need a web version later.
The other is forgetting the paper. CMYK numbers only mean something alongside the profile for a particular press and paper. Ask the printer which profile they want, set it as your working CMYK, and check the result with the proofing tools before you send anything.
Proper CMYK and Lab support is usually found in expensive software. In Paint by Zaweba it sits in the Image menu of a free, offline editor, and it treats your colour numbers with the respect a print job needs.
