Standard color grid (Macbeth, X-Rite) shot at start of each setup — reference for color correction and grading in post. Saves hours.
You photograph the color chart at the beginning of each new lighting situation — this is the most important rule on set. A standardized color pattern plate (usually a Macbeth ColorChecker or similar) is positioned in front of the camera, lit with the same light as the scene, and then captured. You save the image as a reference file. Later in the edit or during color grading, you use this frame to match the camera's raw footage against the known color values of the chart — and thus normalize all shots under that lighting constellation to the same color space.
Why this is not optional: When switching between interior and exterior shots, when changing cameras, or when switching ND filters, the white points and color reproduction can subtly shift. The human eye unconsciously compensates for such differences — your camera does not. If you then place two takes side-by-side in the edit, one shot might suddenly look greenish, the other bluish. With a documented chart, you can trace both back to the exact same reference. This saves you hours in color correction and guarantees consistency over long production days.
In practice, you hold the chart in front of the lens long enough for the camera operator to get at least two to three seconds of footage — don't just flash it through for one second. The assistant marks precisely in the shot log or metadata which timecode or frame contains the chart. During grading, you then navigate to this frame, measure the RGB values against the target values of the plate, and adjust the LUT or curves until the white point is correct and skin tones appear authentic.
Special case for high-contrast scenes: A simple chart may offer little help in extreme backlight situations because it itself will be underexposed. In such cases, you photograph a second chart with additional light on the plate to have a reference in the highlight area. For very dark night shots, you sometimes need a lit chart, sometimes an unlit one — depending on what you need to correct. Practice and experience will show you when one chart is sufficient and when you need variations.