If you just want to see the colors in action, here's some some JavaScript you could run with Deno:
const encoder = new TextEncoder();
function csi(text, ... args) {
const filt = args.filter(xx => xx !== null);
const output = "\x1b[" + filt.join(";") + "m" + text;
Deno.stdout.writeSync(encoder.encode(output));
}
csi("True Color RGB:\n");
for (let ii = 0; ii<32; ++ii) {
for (let jj = 0; jj<64; ++jj) {
const fg_rr = jj*4;
const fg_bb = ii*8;
const fg_gg = (fg_rr + 3*fg_bb)>>2;
const bg_rr = 255 - ii*8;
const bg_gg = 127;
const bg_bb = 255 - jj*4;
csi("+",
38, 2, fg_rr, fg_gg, fg_bb,
48, 2, bg_rr, bg_gg, bg_bb,
);
}
csi("\n");
}