The duration of variations in the potential applied to a single electrical contact, when compared to a reference potential, are measured and employed to command either incrementing or decrementing of an address. The selector can select one of the two color palettes on a character code by character code basis, thereby making it possible to carry out display in a greater number colors on the same screen than the number of colors indicatable by the display color codes stored in the display RAM without increasing the capacity of a font data memory. The two of these channels which best represent the dominant colors of original image are then selected.