1. A color grading tool for producing color-graded display video data from input video data, the color grading tool comprising: a color grading adjustment module operable to color-grade the input video data to yield color-graded video data while displaying the color-graded video data on a color-grading display; and a display conformer configured to modify the color-graded video data for display on a target display and having a greater fidelity range than the color-grading display, by: modeling the display of all or part of the color-graded video data on the color-grading display by a computational display model of the target display to determine one or more modified pixel values that represent the appearance of one or more corresponding pixels of the color-graded video data when displayed on the color-grading display; identifying at least one pixel having values outside the fidelity range of the color-grading display; and modifying the color-graded video data by replacing such pixel values of one or more identified pixels of the color-graded video data for which the identified pixel values are outside a fidelity range of the color-grading display with the corresponding ones of the modified pixel values, wherein the modified pixel values include substitute chromaticities and/or luminances related to the target display, and wherein the display conformer is configured to apply the input video data to the computational display model of the target display, the computational display model configured to determine said substitute chromaticities and/or luminances with which the target display would display such pixels of the input video data which are outside said fidelity range of the color-grading display.