After doing some researches on Google, i've found the 9 bits RGB palette on wiki (
https://en.wikipedia.org/wiki/List_of_monochrome_and_RGB_palettes#9-bit_RGB ) used by a real PCE which uses different values to go up to 255. Do you think you can use it and test it to see if we get better results ? it should be more accurate than the one i've made :
Here is a comparison of the first palette (top left), we can see this new palette rise up the last colors a bit to reach 255 so all colors should be more vivid :
Temper A1 (the one i've made) : (R0,G0,B0) / (R0,G0,B36) / (R0,G0,B72) / (R0,G0,B108) / (R0,G0,B144) / (R0,G0,B180) / (R0,G0,B216) / (R0,G0,B252)
Wiki A1 : (R0,G0,B0) / (R0,G0,B36) / (R0,G0,B72) / (R0,G0,B109) / (R0,G0,B145) / (R0,G0,B182) / (R0,G0,B218) / (R0,G0,B255)