Skin Tone Fix

What causes off skin tones?

Three lighting situations produce the casts this tool fixes most often:

• Warm orange (tungsten light, sunset, candlelight): Indoor incandescent bulbs and golden-hour sunlight push the red channel up and the blue channel down. iPhones under-correct this on purpose to keep skin looking warm, which then reads as “fake-tan orange” on screens.
• Green / yellow-green (fluorescent, low-CRI LED): Office strip lights and cheap LED panels have a strong spike in the green wavelength, which dominates the photo. People look ill.
• Cool magenta (LED, screen glow, over-corrected WB): Some “daylight” LED bulbs spike in magenta. So does sitting in front of a large monitor. Or it's a camera that detected tungsten and over-corrected.

The math, briefly

Sampling a single skin pixel gives us a known “wrong” colour. We compute per-channel scale factors (R, G, B) that, when applied to that sample, transform it into a neutral skin reference at the same luminance. Apply those scale factors to the whole image and the cast disappears.

We modulate that correction by a YCbCr skin-tone mask (centre at Cb=110, Cr=155, smooth falloff to ~55 units away) so the correction lands fully on skin and only 30% on everything else. The 30% global term keeps the photo colour-consistent — a shirt that was a cool blue won't shift to warm orange once the skin is fixed.

When to leave this alone

Two cases where you should not run this tool:

• You want the warm cast. Golden hour, candle-lit dinner, sunset portrait — sometimes the warm colour is the point. Drop the strength slider to zero, or skip this tool entirely.
• The cast is in the makeup, not the light. If the subject is wearing strongly tinted foundation or stage make-up, “correcting” it removes the look. Look for the cast on the eye whites or the neck — those are usually more reliable signals than the cheek.