Daniel Bass of the Blue Sky Rangers invented the "bi-color technique" for multiplexing colors in the same card, chaning them to a completely different hue that even fit the Pantone range of shades. This technique was used in Tower of Doom (different colors look great on CRT screen but RGB emulations tend to actually show the flicker due to higher refresh rates and differences between NTSC phosphors and LCD/LED).
decle reviewed the document and created an approximation of the technique in IntyBASIC!
Media Links:
youtu.be/xheV9xXtXB8
Source Links:
atariage.com/forums/topic/310391-daniel-basss-rainbow-240-additional-colours-for-your-inty/