Selective Colour, Channel Mixer and Photo Filter

Beyond the everyday adjustments, Paint by Zaweba has three colour tools that come from very different traditions. Selective Colour comes from printing, Channel Mixer from the maths of colour channels, and Photo Filter from the coloured glass photographers used to screw onto their lenses. Each does something the others don't, so it's worth knowing when to pick which.

Selective Colour

Selective Colour adjusts the amounts of cyan, magenta, yellow and black within a chosen group of colours. The groups are reds, yellows, greens, cyans, blues and magentas, plus whites, neutrals and blacks.

That's a precise way to fine tune colour. Skin too red? Choose reds and add a touch of cyan. Greens looking yellowish? Choose greens and take out some yellow. Shadows with a colour cast? Choose blacks and correct them without touching the rest.

There are two methods. Relative changes each colour in proportion to how much of that ink it already has, which gives gentler results. Absolute adds or removes a fixed amount, which is stronger. Start with relative.

The neutrals group is a favourite for colour grading, since tinting the neutrals changes a picture's overall mood while leaving strong colours alone.

Channel Mixer

Channel Mixer builds each output channel from percentages of the red, green and blue input channels, plus a constant. In plain terms, it lets you decide how much of each colour feeds into each colour.

Its best known use is black and white. Tick Monochrome and the picture becomes greyscale built from your chosen mix of red, green and blue. A lot of red makes skin smooth and bright and skies darker. A lot of green gives a more natural, even look. Blue tends to be dark and grainy. This is how photographers once used coloured filters on black and white film, and the presets include black and white filter mixes for that reason.

There's also an infrared preset, which mimics the look of infrared photography, with foliage turning pale.

In colour, Channel Mixer can fix strong casts or create bold false colour effects.

Photo Filter

Photo Filter lays a coloured filter over the picture, like holding coloured glass in front of the lens. There are warming and cooling filters, any colour you choose, and sepia, with a density control for strength.

It's the quickest way to warm up a photo taken in cold light, or cool down one that's too orange from indoor lighting. A low density warming filter on a portrait often flatters skin.

Which to use when

For small, targeted colour corrections, use Selective Colour. For black and white conversions with control over how colours become greys, use Channel Mixer. For an overall warm or cool shift, use Photo Filter.

They combine well. A Channel Mixer black and white, followed by a Photo Filter in sepia at low density, gives a toned print look.

Pushing these tools too far

With Channel Mixer in monochrome, keep the total of the three percentages around 100. Much more and the picture blows out; much less and it goes murky.

With Selective Colour, it's easy to overdo absolute mode. If colours start to look poster-like, switch to relative.

With Photo Filter, too much density tints everything heavily. A little goes a long way.

As adjustment layers, all three can be revisited and masked, so you can warm just the subject with Photo Filter or convert only the background to black and white. In Paint by Zaweba, that flexibility is what turns these classic tools into everyday ones.