Color model separating hue, saturation, and brightness — critical in color grading because intensity adjustments don't shift the color itself.
In grading and digital image processing, one works with three separate dimensions of color, not as RGB channels, but as Hue, Saturation, Intensity — the HSI model allows these to be controlled independently. This is the crucial advantage over RGB: you can shift a specific hue without affecting the brightness, or reduce saturation without altering the hue.
In practice, colorists use HSI-based tools such as Hue Curves, Saturation Wheels, or Selective Color Corrections in Resolve, Flame, or similar systems. The workflow is simple: you select a color — let's say flesh tones or a specific sky — isolate it using the Hue Range, and then shift only the hue (H), only the color intensity (S), or only the brightness (I). This works much more intuitively than turning RGB knobs, where every change affects all three channels. For skin tones, you often need only minimal saturation and a slight shift towards yellow — with HSI, you can target this precisely without destroying the highlights or shadows.
A typical scenario: In a daylight scene, you have a sky that has turned too blue-violet. With a Hue Shift in HSI mode, you target only the blues, shifting them towards cyan or pure blue, without changing the brightness or affecting nearby red skin tones. The Intensity element is also central to contrast grading — when you set a DaVinci Qualifier with HSI adjustment, you are primarily working with brightness, not with Lift/Gamma/Gain as in classic Lift-Gamma-Gain wheels.
HSI differs from the HSV (Hue, Saturation, Value) model, which operates in other software contexts — HSI uses Intensity as the true average of all color channels, while HSV works with the highest value. In cinematic grading, you tend to orient yourself towards HSI thinking because it is closer to the perception of light and color. Lumetri in Premiere or curve-based work in Final Cut Pro sometimes use HSL (Lightness instead of Intensity), which is similarly practical.