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HTML

              
                <!--
           |
     \     |     /
       \       /
         ,d8b,           .,
 (')-")_ 88888 ---   ;';'  ';'.
('-  (. ')98P'      ';.,;    ,;
 '-.(PjP)'     \       '.';.'
           |     \
           |
-->

<div class="gradient"></div>
<div class="convert">
  <div class="input">
    <input type="text" id="input" placeholder="B-V Value from -0.4 to 2" />
    <button id="convert">Convert</button>
  </div>
  <div class="output">
  </div>
</div>
              
            
!

CSS

              
                *, *::before, *::after {
  box-sizing: border-box;
}

.gradient {
  display: flex;
  align-items: center;
  justify-content: center;
  flex-direction: row-reverse;
  position: relative;
  height: 100vh;
  width: 100vw;
  
  &:before {
    content: 'B-V Color to Hex';
    display: block;
    position: absolute;
    top: 25px;
    left: 0;
    right: 0;
    width: 100%;
    text-align: center;
    color: white;
    font-size: 48px;
    text-shadow: 1px 1px transparentize(#333, 0.5);
    pointer-events: none;
  }
  
  &:after {
    content: 'Basically finding the color of stars with math, hover with mouse to see values. - Blake Tarter';
    display: block;
    position: absolute;
    top: 90px;
    left: 5%;
    right: 5%;
    width: 90%;
    text-align: center;
    color: white;
    font-size: 22px;
    text-shadow: 1px 1px transparentize(#333, 0.5);
    pointer-events: none;
  }
}

.color {
  flex: 1;
  height: 100vh;
}

.convert {
  position: absolute;
  bottom: 0;
  left: 0;
  right: 0;
  height: 110px;
  width: 100%;
  background: rgba(255, 255, 255, 0.4);
  
  .input {
    width: 50%;
    float: left;
    
    input {
      height: 45px;
      width: 80%;
      margin: 5px auto;
      display: block;
      padding-left: 15px;
    }
    
    button {
      width: 80%;
      background: #333;
      color: white;
      border: none;
      height: 45px;
      margin: 5px auto;
      display: block;
    }
  }
  
  .output {
    width: 50%;
    color: #333;
    line-height: 105px;
    font-size: 64px;
    float: right;
  }
}
              
            
!

JS

              
                // this code is a lot more messy than I usually like, but meh it works.

// color index to temperature in kelvin
function bvToT(bv) {
  let t;
  
  // make sure bv is within its bounds [-0.4, 2] otherwise the math doesnt work
  if (bv < -0.4) {
    bv = -0.4;
  } else if (bv > 2) {
    bv = 2;
  }
  
  // found it online at http://www.wikiwand.com/en/Color_index
  t = 4600 * ((1 / ((0.92 * bv) + 1.7)) + (1 / ((0.92 * bv) + 0.62)));
  
  // console.log('t: ' + t);
  
  return t;
}

// temperature to CIE xyY Colorspace, assume Y is 1
function tToXyy(t) {
  let x, y, Y = 1; // Y is the luminance, I just assume full luminance for sanity
  
  // approximation of CIE xyY (http://www.wikiwand.com/en/CIE_1931_color_space) using https://en.wikipedia.org/wiki/Planckian_locus 
  if (t >= 1667 && t <= 4000) {
    x = (-0.2661239 * (Math.pow(10, 9) / Math.pow(t, 3))) -
        (-0.2343580 * (Math.pow(10, 6) / Math.pow(t, 2))) +
        (0.8776956 * (Math.pow(10, 3) / t)) + 0.179910;
  } else if (t >= 4000 && t <= 25000) {
    x = (-3.0258469 * (Math.pow(10, 9) / Math.pow(t, 3))) +
        (2.1070379 * (Math.pow(10, 6) / Math.pow(t, 2))) +
        (0.2226347 * (Math.pow(10, 3) / t)) + 0.240390;
  }

  if (t >= 1667 && t <= 2222) {
    y = (-1.1063814 * Math.pow(x, 3)) -
        (1.34811020 * Math.pow(x, 2)) +
        (2.18555832 * x) -
         0.20219683;
  } else if (t >= 2222 && t <= 4000) {
    y = (-0.9549476 * Math.pow(x, 3)) -
        (1.37418593 * Math.pow(x, 2)) +
        (2.09137015 * x) -
         0.16748867;
  } else if (t >= 4000 && t <= 25000) {
    y = (3.0817580 * Math.pow(x, 3)) -
        (5.87338670 * Math.pow(x, 2)) +
        (3.75112997 * x) -
         0.37001483;
  }
  
  // console.log('xyY: ' + [x, y, Y]);
  
  return [x, y, Y];
}

// xyY Color space to XYZ, prepping for conversion to linear RGB
function xyYToXyz(xyY) {
  let X, Y, Z,
      x = xyY[0],
      y = xyY[1];

  // X and Z tristimulus values calculated using https://en.wikipedia.org/wiki/CIE_1931_color_space?oldformat=true#CIE_xy_chromaticity_diagram_and_the_CIE_xyY_color_space
  Y = xyY[2];
  X = (y === 0) ? 0 : (x * Y) / y;
  Z = (y === 0) ? 0 : ((1 - x - y) * Y) / y;
  
  // console.log('XYZ: ' + [X, Y, Z]);
  
  return [X, Y, Z];
}

//XYZ color space to linear RGB, finally a format I recognize
function xyzToRgb(xyz) {
  let r, g, b,
      x = xyz[0],
      y = xyz[1],
      z = xyz[2];

  // using matrix from https://www.cs.rit.edu/~ncs/color/t_convert.html#RGB%20to%20XYZ%20&%20XYZ%20to%20RGB
  r = (3.2406 * x) +
      (-1.5372 * y) + 
      (-0.4986 * z);

  g = (-0.9689 * x) +
      (1.8758 * y) +
      (0.0415 * z);

  b = (0.0557 * x) +
      (-0.2040 * y) +
      (1.0570 * z);
  
  // make sure the values didnt overflow
  r = (r > 1) ? 1 : r;
  g = (g > 1) ? 1 : g;
  b = (b > 1) ? 1 : b;
  
  // console.log('rgb: ' + [r, g, b]);

  return [r, g, b];
}

// Im supposed to gamma correct and convert to sRGB but right now it breaks things so TODO: fix this..
function gammaCorrect(rgb) {
  let a = 0.055,
      gamma = 2.2,
      R, G, B,
      r = rgb[0],
      g = rgb[1],
      b = rgb[2];
  
  // using https://en.wikipedia.org/wiki/SRGB?oldformat=true#The_forward_transformation_.28CIE_xyY_or_CIE_XYZ_to_sRGB.29
  /*R = (r <= 0.0031308) ? 12.92 * r : ((1 + r) * Math.pow(r, 1 / 2.2)) - a;
  G = (g <= 0.0031308) ? 12.92 * g : ((1 + g) * Math.pow(g, 1 / 2.2)) - a;
  B = (b <= 0.0031308) ? 12.92 * b : ((1 + b) * Math.pow(b, 1 / 2.2)) - a;*/
  
  /*R = Math.pow(r, 1 / gamma);
  G = Math.pow(g, 1 / gamma);
  B = Math.pow(b, 1 / gamma);*/
  
  R = r;
  G = g / 1.05; // idk but i messed up somewhere and this makes it look better
  B = b;
  
  R = (R > 1) ? 1 : R;
  G = (G > 1) ? 1 : G;
  B = (B > 1) ? 1 : B;
  
  // turn the 0-1 rgb value to 0-255
  return [Math.round(R * 255), Math.round(G * 255), Math.round(B * 255)];
}

// rgb to hex is cake
function rgbToHex(rgb) {
  return '#' + rgb[0].toString(16) + rgb[1].toString(16) + rgb[2].toString(16);
}

// now put it all together!
function bvToRgb(bv) {
  let t, xyY, xyz, rgb, crgb;
  
  t = bvToT(bv);
  
  xyY = tToXyy(t);
  
  xyz = xyYToXyz(xyY);
  
  rgb = xyzToRgb(xyz);
  
  crgb = gammaCorrect(rgb);
  
  return crgb;
}

// loop though and generate an array in a range
function generateGradient(min, max, interval) {
  let i = min, 
      ii = max,
      gradient = [];
  
  for (; i <= ii; i = i + interval) {
    gradient.push({'hex': rgbToHex(bvToRgb(i)), 'bv': i});
  }
  
  return gradient;
}

// now make the gradient array into an array of dom elements
function generateElements(gradient) {
  let i = 0,
      ii = gradient.length,
      elements = [];
  
  for (; i < ii; i++) {
    let tmp = document.createElement('div');
    
    tmp.className = 'color';
    tmp.setAttribute('style', 'background: ' + gradient[i].hex + ';');
    tmp.setAttribute('title', 'B-V: ' + gradient[i].bv.toFixed(2) + ' Hex: ' + gradient[i].hex);
    
    elements.push(tmp);
  }
  
  return elements;
}

// append all the elements to the page
function appendElements(elements) {
  let i = 0,
      ii = elements.length;
  
  for (; i < ii; i++) {
    document.getElementsByClassName('gradient')[0].appendChild(elements[i]);
  }
}

let rgb = bvToRgb(2);

// console.log('Final rgb: ' + rgb);

let hex = rgbToHex(rgb);

// console.log('Hex: ' + hex);

let gradient = generateGradient(-0.4, 2, 0.01);

console.log(gradient);

let elements = generateElements(gradient);

console.log(elements);

appendElements(elements);


// listen for click on the button to convert a value
document.getElementById('convert').addEventListener('click', function(e) {
  e.preventDefault();
  
  let input = document.getElementById('input').value;
  
  document.getElementsByClassName('output')[0].innerHTML = rgbToHex(bvToRgb(input));
});

              
            
!
999px

Console