"javascript convert hsb/hsv color to rgb accurately" Code Answer

from parthik gosar's link in this comment with slight modification to let you enter each value independently or all at once as an object

/* accepts parameters
 * h  object = {h:x, s:y, v:z}
 * or 
 * h, s, v
*/
function hsvtorgb(h, s, v) {
    var r, g, b, i, f, p, q, t;
    if (arguments.length === 1) {
        s = h.s, v = h.v, h = h.h;
    }
    i = math.floor(h * 6);
    f = h * 6 - i;
    p = v * (1 - s);
    q = v * (1 - f * s);
    t = v * (1 - (1 - f) * s);
    switch (i % 6) {
        case 0: r = v, g = t, b = p; break;
        case 1: r = q, g = v, b = p; break;
        case 2: r = p, g = v, b = t; break;
        case 3: r = p, g = q, b = v; break;
        case 4: r = t, g = p, b = v; break;
        case 5: r = v, g = p, b = q; break;
    }
    return {
        r: math.round(r * 255),
        g: math.round(g * 255),
        b: math.round(b * 255)
    };
}

this code expects 0 <= h, s, v <= 1, if you're using degrees or radians, remember to divide them out.

the returned 0 <= r, g, b <= 255 are rounded to the nearest integer. if you don't want this behaviour remove the math.rounds from the returned object.

and the reverse (with less division)

/* accepts parameters
 * r  object = {r:x, g:y, b:z}
 * or 
 * r, g, b
*/
function rgbtohsv(r, g, b) {
    if (arguments.length === 1) {
        g = r.g, b = r.b, r = r.r;
    }
    var max = math.max(r, g, b), min = math.min(r, g, b),
        d = max - min,
        h,
        s = (max === 0 ? 0 : d / max),
        v = max / 255;

    switch (max) {
        case min: h = 0; break;
        case r: h = (g - b) + d * (g < b ? 6: 0); h /= 6 * d; break;
        case g: h = (b - r) + d * 2; h /= 6 * d; break;
        case b: h = (r - g) + d * 4; h /= 6 * d; break;
    }

    return {
        h: h,
        s: s,
        v: v
    };
}

this code will output 0 <= h, s, v <= 1, but this time takes any 0 <= r, g, b <= 255 (does not need to be an integer)

for completeness,

function hsvtohsl(h, s, v) {
    if (arguments.length === 1) {
        s = h.s, v = h.v, h = h.h;
    }
    var _h = h,
        _s = s * v,
        _l = (2 - s) * v;
    _s /= (_l <= 1) ? _l : 2 - _l;
    _l /= 2;

    return {
        h: _h,
        s: _s,
        l: _l
    };
}

function hsltohsv(h, s, l) {
    if (arguments.length === 1) {
        s = h.s, l = h.l, h = h.h;
    }
    var _h = h,
        _s,
        _v;

    l *= 2;
    s *= (l <= 1) ? l : 2 - l;
    _v = (l + s) / 2;
    _s = (2 * s) / (l + s);

    return {
        h: _h,
        s: _s,
        v: _v
    };
}

all of these values should be in the range 0 to 1. for hsl<->rgb go via hsv.