JSFiddle - React, Tailwind, and code Playground

HTML

<div class="card">
<img id="i"...

CSS

.card {
  display: inline-block;
  position: relative;
}
#panel {
  position: absolute;
  bottom: 3px;
  width: 100%;
  padding: 10px 5px;
  box-sizing: border-box;
  color: white;
}

JavaScript

var rgb = getAverageRGB(document.getElementById('i'));
    document.getElementById('panel').style.background = 'linear-gradient(to bottom, rgba('+rgb.r+','+rgb.g+','+rgb.b+', 0) 0%, rgba('+rgb.r+','+rgb.g+','+rgb.b+', 1) 100%)';

function getAverageRGB(imgEl) {
    
    var blockSize = 5, // only visit every 5 pixels
        defaultRGB = {r:0,g:0,b:0}, // for non-supporting envs
        canvas = document.createElement('canvas'),
        context = canvas.getContext && canvas.getContext('2d'),
        data, width, height,
        i = -4,
        length,
        rgb = {r:0,g:0,b:0},
        count = 0;
        
    if (!context) {
        return defaultRGB;
    }
    
    height = canvas.height = imgEl.naturalHeight || imgEl.offsetHeight || imgEl.height;
    width = canvas.width = imgEl.naturalWidth || imgEl.offsetWidth || imgEl.width;
    
    context.drawImage(imgEl, 0, 0);
    
    try {
        data = context.getImageData(0, 0, width, height);
    } catch(e) {
        /* security error, img on diff domain */alert('x');
        return defaultRGB;
    }
    
    length = data.data.length;
    
    while ( (i += blockSize * 4) < length ) {
        ++count;
        rgb.r += data.data[i];
        rgb.g += data.data[i+1];
        rgb.b += data.data[i+2];
    }
    
    // ~~ used to floor values
    rgb.r = ~~(rgb.r/count);
    rgb.g = ~~(rgb.g/count);
    rgb.b = ~~(rgb.b/count);
    
    return rgb;
    
}