A color cast is an unwanted, uniform tint across an image that shifts the overall color balance away from neutral. The most common cause is mismatched white balance: shooting under tungsten light with a daylight white balance setting produces a strong orange cast; shooting under cool LEDs with a daylight setting produces a magenta or green tint. Mixed light sources (a fluorescent overhead combined with daylight from a window, a candle alongside a household incandescent) produce more complex casts that vary across the frame.
Identifying a color cast starts with reference points. Neutral whites and greys in the scene (a wall, a piece of paper, a shirt) should appear neutral in the rendered image. If they show a tint, that tint is the cast, and the correction is to shift the white balance and tint sliders in the opposite direction until the references read neutral. In Lightroom, the white balance eyedropper, dropped on a known-neutral pixel, performs this correction automatically. A grey card or X-Rite ColorChecker placed in the scene during the first frame of a shoot gives a guaranteed reference for batch correction.
Mixed lighting is the trickiest case because no single white balance correction fixes the whole frame. A common indoor scenario: ambient tungsten at roughly 3200K combined with daylight at 5500K through a window. Setting the camera to 3200K produces a blue cast in the window light; setting it to 5500K produces an orange cast everywhere else. Solutions include gelling the strobes or windows with CTO or CTB gels to match color temperatures, using local masking in post to apply different white balance to different regions, or shooting raw so the global white balance can be adjusted non-destructively to favor the most important area of the frame.
Color casts can also originate from sources other than white balance. Reflected light from a strongly colored surface (a green lawn bouncing onto a portrait subject’s face, a red wall lighting the shadow side) creates localized casts that look like skin-tone problems. Old lenses with yellowing thorium glass elements impart a warm cast. Polarizing filters slightly shift color balance. Cyan or magenta casts on the edges of wide-angle lens images can come from lens-sensor color shift on certain mirrorless bodies, particularly when adapting rangefinder lenses, and require lens-profile correction in post.
Casts can also be intentional. A warm cast applied uniformly to a portrait produces the so-called golden hour look; a cool cast produces the cinematic teal-and-orange palette when paired with warm skin tones. The distinction between an unwanted cast and a stylistic color grade is the photographer’s intent. A cast becomes a problem only when it conflicts with the desired look or makes the subject’s skin appear unnaturally tinted.
Prevention is easier than correction. Shooting raw preserves all white balance information for post-processing, since the sensor data is recorded without the white balance baked in. Using a custom white balance set on a grey card under the actual lighting at the shoot is the most accurate method. For mixed lighting, deciding at capture time which light source is the priority (the one that hits the subject) and matching the camera or strobes to that source reduces the casts in the rest of the frame to something manageable. Modern raw vs JPEG workflows give raw a substantial advantage here because JPEG bakes the white balance choice in.