Split toning colours the shadows and highlights separately
Split toning is the oldest trick in colour grading and the one most often overdriven.
Where the colour comes from
A straight black-and-white print in the darkroom was never truly neutral. Fibre-based papers leaned warm; lith printing pushed shadows toward green-black and midtones toward peach. Photographers who wanted deliberate colour in a monochrome image turned to chemical toners — sepia, selenium, gold — each depositing a different metal compound into the silver and shifting the print toward a characteristic hue. Split toning took that one step further: the shadows received one toner and the highlights another, because a warm shadow and a cool highlight, or the reverse, creates a tension the eye reads as atmosphere. Darkroom split-toning chemistry worked by exploiting the fact that highlights and shadows tone at different rates in the same bath, so the duration of the toning controlled which zones were affected.
Digital split toning inherited that logic wholesale, then extended it to colour images. In Lightroom's original HSL-based Split Toning panel — replaced by the Colour Grading wheel in Lightroom Classic 10.0, released in October 2020 — you picked a hue and a saturation for shadows and highlights separately, with a balance slider that biased the boundary between them. Capture One from Phase One has carried its own equivalent for years under Colour Balance. darktable offers the same through its colour balance rgb module. The mechanism differs between programs; the perceptual result is identical in kind.
What the sliders actually do
Hue picks the colour — nothing more. Saturation is the control that matters. At zero, the hue slider does nothing visible; you can run it from 0 to 360 and the image sits unchanged. Start lifting saturation and the chosen hue bleeds into that tonal region. The lighter the pixel in the target zone, the more it accepts the highlight tint; the darker, the more it accepts the shadow tint. The tone curve relationship underneath this means mid-tones are always a blend of both.
The balance control — a single slider offset — determines where that blend boundary sits. Pull it toward shadows and the shadow tint claims more of the lower midtones; push it toward highlights and warmth or coolness creeps further into the upper midtones. Neither end of the balance slider reaches pure black or pure white, which is why split toning affects mood rather than exposure: the darkest darks and the palest whites receive very little, and overall brightness is not shifted.
The colour balance rgb module in darktable ↗ works on the same shadows/midtones/highlights logic but exposes it through three wheels simultaneously, which makes the relationship between zones easier to see and easier to overdo.
How much is too much
Every tutorial screenshot that demonstrates split toning uses an amount that would be wrong in a finished image. That is an instructional problem: exaggeration shows the effect, but it trains the hand to reach too far. A shadow tint that reads as intentional at 15 saturation looks like a broken monitor at 45. The rule Ansel Adams applied to his zone system ↗ holds here in spirit: every tonal decision should serve the image, not announce itself.
The useful test is to toggle the effect off and on at a point when the eye has rested. If the toning is visible as a colour cast in the first second, the saturation is too high. If the image simply reads as cooler, warmer, or more cinematic without the viewer's eye going to find a reason, it is probably right. Portrait highlights almost never benefit from a tint above single digits. Landscape shadows can carry more without drawing attention. The difference is contrast: a face gives the viewer a reference they know, and any colour cast on skin is noticed immediately.