The Palettes
During adventuring there are always two palettes active. Because the Atari had only 16 colors there were not enough colors to simultaneously display the portraits at the top of the screen and the viewport. Especially this was true because the viewport had to be able to be dimmed in low light levels while the portraits had to remain at full intensity. So, while the screen was being painted by the CRT electron gun, the lines were being counted. When the viewport was about to be displayed, the palette was quickly changed. Then, when the viewport was complete, the palette was switched back again to display the text. So sixty times per second (fifty in Europe) the palette was changed twice.
The 'viewport' palette has two colors that are variable, depending on what monsters are present. They are colors number 9 and 10 (starting with zero). When the party enters a new level the list of possible monsters is examined and colors 9 and 10 are modified accordingly.
In the 'graphics.dat' file there is a graphic that CSBwin calls 'graphic #22e'. Among other things, it contains an array of 12-byte structures. The array contains 27 such structures (one for each monster type, I suppose). Currently, this structure is referred to as 'ITEM12' because at the time of its discovery the only thing I knew about it was that it was 12 bytes long. Byte 11 of the structure is used to set colors 9 and 10. If the upper 4-bit nibble of byte 11 is non-zero then it is used to set color 9. If the lower 4-bit nibble of byte 11 is non-zero then it is used to set color 10.