Help with changing lightness of foreColor script

Hello,

I'd like a little help with scripting here: I'm currently using this script I've found in the forums to change the brightness of the current foreground color

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#target photoshop

    var foreColor = new SolidColor;

    foreColorH=app.foregroundColor.hsb.hue; //Initial Value

    foreColorS=app.foregroundColor.hsb.saturation;

    foreColorB=app.foregroundColor.hsb.brightness;

    foreColor.hsb.hue=foreColorH+0;

    if (foreColorB+10 > 100) {foreColor.hsb.brightness=100}

    else {foreColor.hsb.brightness=foreColorB+10};

    foreColor.hsb.saturation=foreColorB+0;

    app.foregroundColor = foreColor;

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Changing this script for -10 brightness does the result below (red arrows), but what I really need is to change the lightness down so the color picker goes straight down, like the blue goes straight up.

Many thanks for any help.