1. An image data update apparatus, the apparatus updating image data showing an image including two areas, anti-aliasing being applied to pixels around a boundary between the two areas, the anti-aliasing smoothing change of color on the boundary, the apparatus comprising: an image data storage means for storing first image data showing a first image, part of which includes a first component and a first area with a predetermined color, the first area separating from each other the first component and a background area of the first component, wherein the anti-aliasing is applied to a pixel around a boundary between the first component and the first area and to a pixel around a boundary between the first area and the background area; and an image data update means for executing an image data update operation having a first step of generating second image data wherein: (i) a second image shown by the second image data generated at the first step of the image data update operation by the image data update means is an image which is generated by changing the first component in the first image into a second component with respect to color, (ii) the first area separating the first component and the background area is a strip-shaped area having a width of at least two pixels or over; and (iii) when the image data update means changes the first component in the first image with respect to the color among the first image data stored in the image data storage means, the anti-aliasing applied to the pixel around the boundary between the first area and the background area is maintained, while anti-aliasing is applied to the pixel around a boundary between the second component and the first area.