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HTML 

              
                <body class="red-and-black">
	
	<!-- Featured Image	 -->
	
	<h2>Featured image</h2>
	<div class="js-module u-flex u-width-600 u-margin-t-40">
		<div class="site-featured-image u-width-600">
			<figure class="post-thumbnail a-filter-child-img">
				<img src="http://localhost/c4aa/wp-content/uploads/2020/02/IMG_6019-1568x701.jpg" class="attachment-post-thumbnail size-post-thumbnail wp-post-image" alt="" srcset="http://localhost/c4aa/wp-content/uploads/2020/02/IMG_6019-1568x701.jpg 1568w, http://localhost/c4aa/wp-content/uploads/2020/02/IMG_6019-580x259.jpg 580w, http://localhost/c4aa/wp-content/uploads/2020/02/IMG_6019-1400x626.jpg 1400w, http://localhost/c4aa/wp-content/uploads/2020/02/IMG_6019-768x343.jpg 768w, http://localhost/c4aa/wp-content/uploads/2020/02/IMG_6019-1536x687.jpg 1536w, http://localhost/c4aa/wp-content/uploads/2020/02/IMG_6019-2048x915.jpg 2048w" sizes="(max-width: 1568px) 100vw, 1568px" width="1568" height="701">					
			</figure>
			<div class="entry-header">		
				<h1 class="entry-title">image filters test</h1>
			</div><!-- .entry-header -->
		</div>
		<div class="js-button-container u-flex u-flex-direction-column">
			<button class="beige-and-red">beige-and-red</button>
			<button class="red-and-black">red-and-black</button>
			<button class="beige-and-black">beige-and-black</button>
			<button class="beige-and-grey">beige-and-grey</button>
			<button class="beige-and-grey-vintage">beige-and-grey-vintage</button>
</div>
	</div>
	
	
	<!-- Image Block	 -->
	
	<h2>Inline block images</h2>
	<div class="js-module u-width-600 u-flex">
		<div class="a-filter-child-img">
			<img src="https://live.staticflickr.com/1960/45670544451_05dbf59644_n.jpg" alt="">
		</div>
		<div class="js-button-container u-flex u-flex-direction-column">
			<button class="beige-and-red">beige-and-red</button>
			<button class="red-and-black">red-and-black</button>
		<button class="beige-and-black">beige-and-black</button>
			<button class="beige-and-grey">beige-and-grey</button>
			<button class="beige-and-grey-vintage">beige-and-grey-vintage</button>
		</div>
	</div>
	<div class="js-module u-width-600 u-flex u-margin-t-40">
		<div class="a-filter-child-img">
			<img src="https://live.staticflickr.com/4754/26537562208_100527f564.jpg" alt="">
		</div>
		<div class="js-button-container u-flex u-flex-direction-column">
			<button class="beige-and-red">beige-and-red</button>
			<button class="red-and-black">red-and-black</button>
		<button class="beige-and-black">beige-and-black</button>
			<button class="beige-and-grey">beige-and-grey</button>
			<button class="beige-and-grey-vintage">beige-and-grey-vintage</button>
		</div>
	</div>

	
	<!-- Cover Block	 -->

	<h2>WP cover block</h2>
	<div class="js-module u-flex u-width-600 u-margin-t-40">
		<div class="a-filter-cover-img wp-block-cover has-background-dim-30 has-background-dim" style="background-image:url(http://backtobasics.c4aa.org/wp-content/uploads/2019/05/NYC_Brooklyn_Musum_OSF_Arts_Exchange_SCA_2NYC_Met_Museum_NY_OSF_SCA_201804-8381.jpg)"><div class="wp-block-cover__inner-container">
		<p class="has-text-align-center has-huge-font-size"> The C4AA offers training, advising, research, and resources.</p>
	</div>	
		
	</div>
		<div class="js-button-container u-flex u-flex-direction-column">
			<button class="beige-and-red">beige-and-red</button>
			<button class="red-and-black">red-and-black</button>
		<button class="beige-and-black">beige-and-black</button>
			<button class="beige-and-grey">beige-and-grey</button>
			<button class="beige-and-grey-vintage">beige-and-grey-vintage</button>
		</div>
	</div>
</body>
!

CSS 

              
                
/* Duotone Defaults */
:root {
	--color-brandBeige: #f2f1ed; /* Fallback */
	--color-brandBeige: hsla(48, 16%, 94%, 1);
	--color-brandBlack: #292724; /* Fallback */
	--color-brandBlack: hsla(36, 6%, 5%, 0.9);
	--color-brandBlue: #005eb8; /* Fallback */
	--color-brandBlue: hsla(209, 100%, 34%, 0.9);
	--color-brandGrey: #7b736c; /* Fallback */
	--color-brandGrey: hsla(28, 6%, 42%, 0.9);
	--color-brandRed: #da2f3b; /* Fallback */
	--color-brandRed: hsla(356, 80%, 45%, 0.9);
	--color-brandWhite: #fafaf9; /* Fallback */
	--color-brandWhite: hsla(60, 9%, 98%, 0.9);
	--color-brandYellow: #e1cd00; /* Fallback */
	--color-brandYellow: hsla(55, 100%, 46%, 0.9);

	/* Default Colors */
	--duotone-default-base: var(--color-brandBeige);
	--duotone-default-foreground: var(--color-brandRed);

	/* Default Image Treatment */
	--duotone-base: var(--duotone-default-base);
	--duotone-bg-blend: multiply;
	--duotone-blur: 0px;
	--duotone-fg-blend: screen;
	--duotone-foreground: var(--duotone-default-foreground);
	--duotone-opacity: 1;
	--duotone-spacing: 1%;
}

.a-filter-child-img {
	background-color: var(--duotone-base);
	overflow: hidden;
	padding: var(--duotone-spacing);
	position: relative;
	transition: background-color 250ms ease;
}

.a-filter-child-img img {
	filter: grayscale(100%) contrast(1) blur(var(--duotone-blur));
	mix-blend-mode: var(--duotone-bg-blend);
	object-fit: cover;
	opacity: var(--duotone-opacity);
	position: relative;
	width: 100%;
	transition: mix-blend-mode filter opacity 250ms ease-in-out;
}

.a-filter-child-img::before {
	background-color: var(--duotone-foreground);
	bottom: 0;
	content: "";
	height: 100%;
	left: 0;
	mix-blend-mode: var(--duotone-fg-blend);
	position: absolute;
	right: 0;
	top: 0;
	width: 100%;
	z-index: 1;
}

/* Hover state */

.a-filter-child-img:hover {
	background-color: initial;
	opacity: 1;
}

.a-filter-child-img:hover::before {
	background-color: initial;
}

.a-filter-child-img:hover img {
	filter: none;
	mix-blend-mode: normal;
	opacity: 1;
}

/* Themes */

.red-and-black {
	--duotone-base: var(--color-brandRed);
	--duotone-foreground: var(--color-brandBlack);
	--duotone-bg-blend: multiply;
	--duotone-fg-blend: screen;
	--duotone-opacity: 0.96;
}

.beige-and-black {
	--duotone-base: var(--color-brandBeige);
	--duotone-foreground: var(--color-brandBlack);
	--duotone-bg-blend: multiply;
	--duotone-fg-blend: lighten;
	--duotone-opacity: 1;
}

.beige-and-red {
	--duotone-base: var(--color-brandBeige);
	--duotone-foreground: var(--color-brandRed);
	--duotone-bg-blend: multiply;
	--duotone-fg-blend: screen;
	--duotone-opacity: 1;
}

.beige-and-grey {
	--duotone-base: var(--color-brandBeige);
	--duotone-foreground: var(--color-brandGrey);
	--duotone-bg-blend: multiply;
	--duotone-fg-blend: screen;
	--duotone-opacity: 1;
}

.beige-and-grey-vintage {
	--duotone-base: var(--color-brandBeige);
	--duotone-foreground: var(--color-brandBlack);
	--duotone-bg-blend: color-burn;
	--duotone-fg-blend: screen;
	--duotone-opacity: 0.69;
}

/* Utils */

button {
	white-space: nowrap;
}

.u-width-600 {
	width: 600px;
}

.u-flex {
	display: flex;
}

.u-flex-direction-column {
	flex-direction: column;
}

.u-margin-t-40 {
	margin-top: 40px;
}

/* Basic WP cover block styles */

.wp-block-cover {
	background-attachment: fixed;
	color: white;
}

.wp-block-cover.has-background-dim::before {
	content: "";
	position: absolute;
	top: 0;
	left: 0;
	bottom: 0;
	right: 0;
	background-color: inherit;
	opacity: 0.5;
	z-index: 1;	
}

.wp-block-cover__inner-container {
	z-index: 1;	
}

.wp-block-cover {
	background-color: #000;
	position: relative;
	background-size: cover;
	background-position: 50%;
	min-height: 430px;
	width: 100%;
	margin: 0 0 1.5em;
	display: flex;
	justify-content: center;
	align-items: center;
	overflow: hidden;
}

.a-filter-cover-img {
	background-color: var( --duotone-base );
	background-blend-mode: var( --duotone-bg-blend );
/* 	Could do stuff like this: */
/* 	filter: grayscale(30%) sepia(30%) contrast(2) brightness(1.5); */
}

.a-filter-cover-img::before {
	background-color: var( --duotone-foreground ) !important;
}


/* Misc */

img {
	max-width: 100%;
	height: auto;
}
!

JS 

              
                document.querySelectorAll( '.js-module' ).forEach( ( el ) => {
	let btnListener = el.querySelector( '.js-button-container' );

	btnListener.addEventListener( 'click', ( e ) => {
		
		if ( e.target.nodeName == 'BUTTON' ) {
			if ( el.querySelector( '.a-filter-child-img' ) ) {
				el.querySelector( '.a-filter-child-img' ).setAttribute( 'class', 'a-filter-child-img ' + e.target.textContent );
			}
			
			if ( el.querySelector( '.a-filter-cover-img' ) ) {
				el.querySelector( '.a-filter-cover-img' ).setAttribute( 'class', 'wp-block-cover wp-block-cover has-background-dim a-filter-cover-img ' + e.target.textContent );
			}
		}
	});
})
!
999px

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