pencolor --Command, one input. Abbreviation: penc
     This command applies only to the display turtle. Its effect is
     different depending on how each type of terminal supports color. For
     the Atari, the input must be an integer between 0 and 6. An input of
     0 enters black-and-white display mode (which is the turtle's initial
     mode), in which lines are as thin as possible but there is no control
     of color. Any other input selects color mode, in which lines are
     twice as thick, so the effective size of the screen is smaller, but
     colors can be used. There are, in color mode, three possible pen
     colors, numbered 1 to 3. There are 256 possible colors, but only
     three can be on the screen at a time; the setcolor command is used to
     decide which pen draws in which actual color. If the input is 4, 5,
     or 6, the color is that of pen 1, 2, or 3, respectively, but lines are
     drawn in "fill mode": for each point inked, all points to its right
     are also inked until a point is reached which was already inked. On
     the Gigi, there is only one mode, and there is no loss of resolution
     in using color. The input must be between 0 and 7; 0 means black, 7
     means white. The ADM, Tektronix, and Sun displays do not have multi-
     color drawing.