Xterm 256color mode color chart, organised into sections.

xterm-256color.svg

xterm-256color.yaml

# render as two 8 cell rows
--- 
:xterm16: 
- ["000", "#000000"]
- ["001", "#800000"]
- ["002", "#008000"]
- ["003", "#808000"]
- ["004", "#000080"]
- ["005", "#800080"]
- ["006", "#008080"]
- ["007", "#c0c0c0"]
- ["008", "#808080"]
- ["009", "#ff0000"]
- ["010", "#00ff00"]
- ["011", "#ffff00"]
- ["012", "#0000ff"]
- ["013", "#ff00ff"]
- ["014", "#00ffff"]
- ["015", "#ffffff"]

# render as two 12 cell rows 
:xtermGreyscale:  
- ["232", "#080808"]
- ["233", "#121212"]
- ["234", "#1c1c1c"]
- ["235", "#262626"]
- ["236", "#303030"]
- ["237", "#3a3a3a"]
- ["238", "#444444"]
- ["239", "#4e4e4e"]
- ["240", "#585858"]
- ["241", "#626262"]
- ["242", "#6c6c6c"]
- ["243", "#767676"]
- ["255", "#eeeeee"]
- ["254", "#e4e4e4"]
- ["253", "#dadada"]
- ["252", "#d0d0d0"]
- ["251", "#c6c6c6"]
- ["250", "#bcbcbc"]
- ["249", "#b2b2b2"]
- ["248", "#a8a8a8"]
- ["247", "#9e9e9e"]
- ["246", "#949494"]
- ["245", "#8a8a8a"]
- ["244", "#808080"]
 
# render as 3 table blocks.
# each block rendered top -> bottom, left -> right
# 6 cells down, 12 cells across. (modular arithmetic time!)
:xterm256: 
- ["016", "#000000"]
- ["017", "#00005f"]
- ["018", "#000087"]
- ["019", "#0000af"]
- ["020", "#0000d7"]
- ["021", "#0000ff"]
- ["022", "#005f00"]
- ["023", "#005f5f"]
- ["024", "#005f87"]
- ["025", "#005faf"]
- ["026", "#005fd7"]
- ["027", "#005fff"]
- ["028", "#008700"]
- ["029", "#00875f"]
- ["030", "#008787"]
- ["031", "#0087af"]
- ["032", "#0087d7"]
- ["033", "#0087ff"]
- ["034", "#00af00"]
- ["035", "#00af5f"]
- ["036", "#00af87"]
- ["037", "#00afaf"]
- ["038", "#00afd7"]
- ["039", "#00afff"]
- ["040", "#00d700"]
- ["041", "#00d75f"]
- ["042", "#00d787"]
- ["043", "#00d7af"]
- ["044", "#00d7d7"]
- ["045", "#00d7ff"]
- ["046", "#00ff00"]
- ["047", "#00ff5f"]
- ["048", "#00ff87"]
- ["049", "#00ffaf"]
- ["050", "#00ffd7"]
- ["051", "#00ffff"]
- ["082", "#5fff00"]
- ["083", "#5fff5f"]
- ["084", "#5fff87"]
- ["085", "#5fffaf"]
- ["086", "#5fffd7"]
- ["087", "#5fffff"]
- ["076", "#5fd700"]
- ["077", "#5fd75f"]
- ["078", "#5fd787"]
- ["079", "#5fd7af"]
- ["080", "#5fd7d7"]
- ["081", "#5fd7ff"]
- ["070", "#5faf00"]
- ["071", "#5faf5f"]
- ["072", "#5faf87"]
- ["073", "#5fafaf"]
- ["074", "#5fafd7"]
- ["075", "#5fafff"]
- ["064", "#5f8700"]
- ["065", "#5f875f"]
- ["066", "#5f8787"]
- ["067", "#5f87af"]
- ["068", "#5f87d7"]
- ["069", "#5f87ff"]
- ["058", "#5f5f00"]
- ["059", "#5f5f5f"]
- ["060", "#5f5f87"]
- ["061", "#5f5faf"]
- ["062", "#5f5fd7"]
- ["063", "#5f5fff"]
- ["052", "#5f0000"]
- ["053", "#5f005f"]
- ["054", "#5f0087"]
- ["055", "#5f00af"]
- ["056", "#5f00d7"]
- ["057", "#5f00ff"]
- ["093", "#8700ff"]
- ["092", "#8700d7"]
- ["091", "#8700af"]
- ["090", "#870087"]
- ["089", "#87005f"]
- ["088", "#870000"]
- ["099", "#875fff"]
- ["098", "#875fd7"]
- ["097", "#875faf"]
- ["096", "#875f87"]
- ["095", "#875f5f"]
- ["094", "#875f00"]
- ["105", "#8787ff"]
- ["104", "#8787d7"]
- ["103", "#8787af"]
- ["102", "#878787"]
- ["101", "#87875f"]
- ["100", "#878700"]
- ["111", "#87afff"]
- ["110", "#87afd7"]
- ["109", "#87afaf"]
- ["108", "#87af87"]
- ["107", "#87af5f"]
- ["106", "#87af00"]
- ["117", "#87d7ff"]
- ["116", "#87d7d7"]
- ["115", "#87d7af"]
- ["114", "#87d787"]
- ["113", "#87d75f"]
- ["112", "#87d700"]
- ["123", "#87ffff"]
- ["122", "#87ffd7"]
- ["121", "#87ffaf"]
- ["120", "#87ff87"]
- ["119", "#87ff5f"]
- ["118", "#87ff00"]
- ["159", "#afffff"]
- ["158", "#afffd7"]
- ["157", "#afffaf"]
- ["156", "#afff87"]
- ["155", "#afff5f"]
- ["154", "#afff00"]
- ["153", "#afd7ff"]
- ["152", "#afd7d7"]
- ["151", "#afd7af"]
- ["150", "#afd787"]
- ["149", "#afd75f"]
- ["148", "#afd700"]
- ["147", "#afafff"]
- ["146", "#afafd7"]
- ["145", "#afafaf"]
- ["144", "#afaf87"]
- ["143", "#afaf5f"]
- ["142", "#afaf00"]
- ["141", "#af87ff"]
- ["140", "#af87d7"]
- ["139", "#af87af"]
- ["138", "#af8787"]
- ["137", "#af875f"]
- ["136", "#af8700"]
- ["135", "#af5fff"]
- ["134", "#af5fd7"]
- ["133", "#af5faf"]
- ["132", "#af5f87"]
- ["131", "#af5f5f"]
- ["130", "#af5f00"]
- ["129", "#af00ff"]
- ["128", "#af00d7"]
- ["127", "#af00af"]
- ["126", "#af0087"]
- ["125", "#af005f"]
- ["124", "#af0000"]
- ["160", "#d70000"]
- ["161", "#d7005f"]
- ["162", "#d70087"]
- ["163", "#d700af"]
- ["164", "#d700d7"]
- ["165", "#d700ff"]
- ["166", "#d75f00"]
- ["167", "#d75f5f"]
- ["168", "#d75f87"]
- ["169", "#d75faf"]
- ["170", "#d75fd7"]
- ["171", "#d75fff"]
- ["172", "#d78700"]
- ["173", "#d7875f"]
- ["174", "#d78787"]
- ["175", "#d787af"]
- ["176", "#d787d7"]
- ["177", "#d787ff"]
- ["178", "#dfaf00"]
- ["179", "#dfaf5f"]
- ["180", "#dfaf87"]
- ["181", "#dfafaf"]
- ["182", "#dfafdf"]
- ["183", "#dfafff"]
- ["184", "#dfdf00"]
- ["185", "#dfdf5f"]
- ["186", "#dfdf87"]
- ["187", "#dfdfaf"]
- ["188", "#dfdfdf"]
- ["189", "#dfdfff"]
- ["190", "#dfff00"]
- ["191", "#dfff5f"]
- ["192", "#dfff87"]
- ["193", "#dfffaf"]
- ["194", "#dfffdf"]
- ["195", "#dfffff"]
- ["226", "#ffff00"]
- ["227", "#ffff5f"]
- ["228", "#ffff87"]
- ["229", "#ffffaf"]
- ["230", "#ffffdf"]
- ["231", "#ffffff"]
- ["220", "#ffdf00"]
- ["221", "#ffdf5f"]
- ["222", "#ffdf87"]
- ["223", "#ffdfaf"]
- ["224", "#ffdfdf"]
- ["225", "#ffdfff"]
- ["214", "#ffaf00"]
- ["215", "#ffaf5f"]
- ["216", "#ffaf87"]
- ["217", "#ffafaf"]
- ["218", "#ffafdf"]
- ["219", "#ffafff"]
- ["208", "#ff8700"]
- ["209", "#ff875f"]
- ["210", "#ff8787"]
- ["211", "#ff87af"]
- ["212", "#ff87df"]
- ["213", "#ff87ff"]
- ["202", "#ff5f00"]
- ["203", "#ff5f5f"]
- ["204", "#ff5f87"]
- ["205", "#ff5faf"]
- ["206", "#ff5fdf"]
- ["207", "#ff5fff"]
- ["196", "#ff0000"]
- ["197", "#ff005f"]
- ["198", "#ff0087"]
- ["199", "#ff00af"]
- ["200", "#ff00df"]
- ["201", "#ff00ff"]

# Local Variables:
# eval: (when (fboundp 'rainbow-mode) (rainbow-mode +1))
# End:

The (r, g, b) tuples of the xterm256 hex codes are the product of {0, 95, 135, 175, 215, 255}. There are 6^3 tuples. The pattern is broken towards the end, though, with 223 appearing in place of 215. You have xterm 200, for example, as '#ff00df'/(255, 0, 223). Should this actually be '#ff00d7'/(255, 0, 215) or does the pattern actually switch from 215 to 223 at some point?

See also https://github.com/sindresorhus/xterm-colors which uses d7 instead of df throughout. (Disclaimer: I'm just a random visitor Googl/DDGing around and have no clue whatsoever.)

After more digging – the df values are indeed wrong.

See the xterm source at https://invisible-island.net/xterm/#download, file 256colres.pl, which contains the formula for the sequence of color values used in xterm for each of the R/G/B components: (0=00, 95=5f, 135=87, 175=af, 215=d7, 255=ff).