Any suggestions besides micro and helix?

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");
    }

Apple’s own Terminal.app supports 24 bit color in Tahoe. Not sure if you were already aware, which sparked the question, or not.

When you say Terminal app, do you really mean shell commands? i.e. do you want to run something in Terminal.app that will demonstrate the expanded colour range?

Try btop, a resource monitor with true colour support and 37 builtin themes.

iterm, it's the best

If I am not mistaken, Alacritty does [1]. I believe iTerm2 and Kitty support 24 bit color too.

[1] https://github.com/alacritty/alacritty