Thanks for your valuable input.
I had forgotten about the existence of the palette technique.
The 54540C has a 3-bit color depth (TOTAL 512color) for each RGB color, if we only look at the wires to the LCD. However, as long as it is observed in test mode, etc., it always uses R0=R1=R2, G0=G1=2, and B0=B1=B2, in other words, only 8 colors are used.
The original LCD panel LTM08C015KA is 3-bit RGB each, while the NL6448BC26 used for this replacement is 6-bit RGB each, so it is necessary to convert the color depth if necessary.
The hardware for color depth conversion is already ready, but I did not want to add extra circuitry for a situation where it will never be used.
If there is a mode such as eye pattern measurement that overlays waveforms like later generation oscilloscopes, there is a possibility of using more than 7 colors at the same time.
If I can get information from someone that the 54540C uses something other than the basic 8 colors, I will incorporate a color depth conversion circuit, otherwise I will just use the current simple connection.
Thank you.