1. An image processing unit comprising: a monotone image generator which generates a monotone image from a color image, the monotone image having a highest brightness portion and a lowest brightness portion, and no portion with a brightness that is higher or lower than in the highest and lowest brightness portions, respectively; and a color reducer which reduces a color by reducing chroma in accordance with brightness, wherein the color reducer adjusts a tone of the monotone image generated by the monotone image generator by reducing a color of a high brightness portion of the monotone image, by reducing a color of a low brightness portion of the monotone image, and by reducing a color of the high brightness portion of the monotone image at a higher level than the low brightness portion of the monotone image in response to a same input color difference, such that the level of the color reduction is higher in the high brightness portion of the monotone image than in the low brightness portion of the monotone image, the high brightness portion of the monotone image includes the highest brightness portion, and the low brightness portion includes the lowest brightness portion, the reducing of the color of the low brightness portion includes decreasing the level of color reduction as brightness increases from the lowest brightness portion, such that the level of color reduction is higher in the lowest brightness portion than in another portion of the low brightness portion, and the reducing of the color of the high brightness portion includes increasing the level of color reduction as brightness increases, and wherein the level of color reduction in the highest brightness portion is higher than in the lowest brightness portion.