The atari terminal has an option to set the character size and the screen colors. The driver expects a space separated list the char size and maximal 16 3 digit hex numbers where each digit represents RED, GREEN and BLUE (in that order). The range of 0-15 is scaled to whatever color range the screen actually has. On a normal ST screen, odd and even intensities are the same.
Examples:
set terminal atari 4 # (use small (6x6) font)
set terminal atari 6 0 # (set monochrome screen to white on black)
set terminal atari 13 0 fff f00 f0 f ff f0f ff0
# (set first eight colors to black, white, green, blue, cyan, \
purple, and yellow and use large font (8x16).)
Additionally, if an environment variable GNUCOLORS exists, its contents are interpreted as an options string, but an explicit terminal option takes precedence.