This paper describes a new method for assisting the coloring work to achieve good color harmony in documents that contain color diagrams, figures, or images. Generally, good quality for a color document is achieved by color-element unification. Tone, a color element, is often used to arrange color harmony in a color document. A tone model is proposed that describes the color harmonization concept. An approximation formula is developed to describe the tone model. It enables simple correspondence between color coordinates and their tones. Using the tone model, users can generate a color palette that consists of colors whose subjective impression is unified. This palette is provided for coloring work, Because each of these colors has a uniform element in common, a color document, which is painted by colors from one palette, achieves the color harmonization. The tone model also provides another kind of human interface that allows the users to intuitively select a color by using the adjectives that give us the impression of color variations. Emphasis on a diagram part by selecting a color with a different tone is easily visualized. Using the prototype system, the time for coloring work was significantly reduced.