You can get even more than 13 simultaneous colours by writing to the palette mid-scanline. With this method it is possible to display all 410 available colours in one frame.
Neat. I was also wondering if maybe it was possible to switch between two colors based on a certain ratio of frames, faster than the CRT phosphors to fully switch colors (or atleast faster than the eye could detect the swapping), such that it looks like a fractional blend between two colors based on that ratio. I remember doing this trick with the TI-85 calculator to give any degree of greyscale even though the LCD was only black and white.
