Spot colour channels for print jobs
Add spot colour channels with an ink colour and solidity in Paint by Zaweba, see them on the canvas, and write them into TIFF and PSD files.
Most printing builds colours from four inks. Some jobs need a fifth: a special ink mixed to an exact shade, a metallic, a white underprint, or a varnish that goes only on certain areas. Printers call these spot colours, and the artwork for each one is supplied as its own channel. Paint by Zaweba lets you add and paint those channels alongside the picture.
Adding a spot channel
A spot channel has two settings. The ink colour is how it will be shown to you on screen, so pick something close to the real ink. Solidity describes how the ink behaves on top of what's beneath it. At 0 percent it multiplies like a transparent ink, letting the colours below show through and darkening them. At 100 percent it covers completely, like an opaque ink. Values in between sit between those two.
Once added, the channel appears in the Channels panel along with the colour channels and any alpha channels. You paint on it like any channel: where it's dark there will be ink, where it's light there won't be, and greys give lighter coverage.
What you see and what is saved
A spot channel shows on the canvas only. It is not part of exports or flattening. That's deliberate. The spot ink isn't one of the picture's colours, so mixing it into the RGB or CMYK pixels would ruin the separation the printer needs. Instead, the canvas gives you a preview of how the ink will look over the image, while the channel itself stays separate.
It is saved in the app's own .ptx documents, so you can close and reopen the file and carry on. For handing work to a printer or another program, TIFF and PSD can carry spot channels when you tick the Extra channels option on export.
Typical jobs
A logo printed in a brand's exact ink colour on top of a photo is the classic case. Put the photo in the colour channels and paint the logo in a spot channel. Another is a spot varnish: a gloss finish on just the product in a brochure picture. Make a selection of the product, fill it on a spot channel, and the printer has exactly the area to varnish.
White underprint for printing on clear or coloured material works the same way, as does a metallic ink for a heading or border.
Practical advice for spot work
Set solidity to match the real ink. A transparent ink previewed as opaque, or the other way round, will give you a wrong idea of the final look. If you're not sure how the ink behaves, ask the printer.
Use selections to make clean shapes. Select > Colour Range, saved selections and Quick Mask all help you build the area, and then a fill on the spot channel gives crisp edges.
Surprises at the printer
Expecting a spot channel to appear in a JPEG or PNG is the common surprise. It won't, by design. Use TIFF or PSD with Extra channels for print hand-off.
Flattening and expecting the spot ink to be merged in is the other. Flattening leaves the spot channel out of the result. If you want a picture with the spot colour mixed in for an on-screen proof, that is a separate job from the print file.
Spot channels are a narrow tool, but when a print job calls for them it's a relief that Paint by Zaweba handles them in the same document as the picture, with an honest preview on screen.
