.testclass {
    background-image: url("../images/image.jpg"), linear-gradient(rgba(0,0,0,0.5),rgba(0,0,0,0.5));
    background-blend-mode: overlay;
}

/*
	Or if you like to add a transparent gray layer over your image or div background
	you can use "filter:"
	example:
*/ 

.card img{
    width: 100%;
    height: 100%;
    object-fit: cover;
    border: 10px solid #fff;
    box-shadow: 0px 2px 5px rgba(0, 0, 0, 0.1);
    border-radius: 5px;
    transition: 0.5s;
}

.card img:hover{
    transform: scale(1.02);
    filter: blur(2px) grayscale(50%);
}

html {
  min-height:100%;
  background:linear-gradient(0deg, rgba(255, 0, 150, 0.3), rgba(255, 0, 150, 0.3)), url(http://lorempixel.com/800/600/nature/2);
  background-size:cover;
}

html {
  background:url(http://lorempixel.com/800/600/nature/2);
  background-size:cover;
  box-shadow:inset 0 0 0 2000px rgba(255, 0, 150, 0.3);
}

.header {
  background: rgba(0, 0, 0, 0.5); /* Black color with 50% alpha/opacity */
}

.image:before{
    content: "";
    background: rgba(0,0,0,.6);
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 24px;
}

<div id="element-with-background-image">
   <div id="color-overlay"></div>
   ...
</div>