1. A method of converting a negative image recorded on a negative image-recording medium into a positive image using a predetermined image-conversion rule said method comprising the steps of: (a) determining a gray condition defining achromatic colors on said negative image-recording medium; (b) determining a first combination of color components in a brightest region on said negative image; (c) determining whether or not said first combination of color components satisfies said gray condition; (d) determining said image-conversion rule using said first combination of color components if said first combination of color components satisfies said gray condition; (e) determining said image-conversion rule using an alternative combination of color components if said first combination of color components does not satisfy said gray condition; and (f) converting said negative image into a positive image using said image conversion rule; wherein the step (a) comprises the step of: (a-1) determining a gray reference line in a color coordinate space representing said gray condition; and the step (c) comprises the step of: (c-1) determining whether or not said first combination of color components is in a neighborhood of said gray reference line; wherein the step (c-1) comprises the step of: extracting a first color component from said first combination of color components to obtain a first value; extracting other color components from said first combination of color components; applying said other color components to gray reference line to obtain other values; determining a maximum value and a minimum value among said first value and said other values; calculating a value reflecting a difference between said maximum value and said minimum value; and comparing said value with a predetermined threshold value.