diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -5750,6 +5750,7 @@
     'PhutilRemarkupEscapeRemarkupRule' => 'infrastructure/markup/markuprule/PhutilRemarkupEscapeRemarkupRule.php',
     'PhutilRemarkupEvalRule' => 'infrastructure/markup/markuprule/PhutilRemarkupEvalRule.php',
     'PhutilRemarkupHeaderBlockRule' => 'infrastructure/markup/blockrule/PhutilRemarkupHeaderBlockRule.php',
+    'PhutilRemarkupHexColorCodeRule' => 'infrastructure/markup/markuprule/PhutilRemarkupHexColorCodeRule.php',
     'PhutilRemarkupHighlightRule' => 'infrastructure/markup/markuprule/PhutilRemarkupHighlightRule.php',
     'PhutilRemarkupHorizontalRuleBlockRule' => 'infrastructure/markup/blockrule/PhutilRemarkupHorizontalRuleBlockRule.php',
     'PhutilRemarkupHyperlinkEngineExtension' => 'infrastructure/markup/markuprule/PhutilRemarkupHyperlinkEngineExtension.php',
@@ -12637,6 +12638,7 @@
     'PhutilRemarkupEscapeRemarkupRule' => 'PhutilRemarkupRule',
     'PhutilRemarkupEvalRule' => 'PhutilRemarkupRule',
     'PhutilRemarkupHeaderBlockRule' => 'PhutilRemarkupBlockRule',
+    'PhutilRemarkupHexColorCodeRule' => 'PhabricatorRemarkupCustomInlineRule',
     'PhutilRemarkupHighlightRule' => 'PhutilRemarkupRule',
     'PhutilRemarkupHorizontalRuleBlockRule' => 'PhutilRemarkupBlockRule',
     'PhutilRemarkupHyperlinkEngineExtension' => 'Phobject',
diff --git a/src/infrastructure/markup/markuprule/PhutilRemarkupHexColorCodeRule.php b/src/infrastructure/markup/markuprule/PhutilRemarkupHexColorCodeRule.php
new file mode 100644
--- /dev/null
+++ b/src/infrastructure/markup/markuprule/PhutilRemarkupHexColorCodeRule.php
@@ -0,0 +1,55 @@
+<?php
+
+final class PhutilRemarkupHexColorCodeRule
+ extends PhabricatorRemarkupCustomInlineRule {
+
+  public function getPriority() {
+    return 1000.0;
+  }
+
+
+  protected function contrastingColor($color_code) {
+    $match = ltrim($color_code, '#');
+    $colors_hex = str_split($match, 2);
+    list($r, $g, $b) = array_map('hexdec', $colors_hex);
+    // Calculation adapted from Myndex, CC BY-SA 4.0
+    // https://stackoverflow.com/a/69869976
+    $y = pow((double)$r / 255.0, 2.2) * 0.2126 +
+      pow((double)$g / 255.0, 2.2) * 0.7152 +
+      pow((double)$b / 255.0, 2.2) * 0.0722;
+
+    return ($y < 0.34) ? 'white' : 'black';
+  }
+
+  public function apply($text) {
+    if ($this->getEngine()->isTextMode()) {
+      return $text;
+    }
+    return preg_replace_callback(
+      '@\B{(#[0-9a-fA-F]{6,})}\B@',
+      array($this, 'markupHexColorCodedText'),
+      $text);
+  }
+
+  protected function markupHexColorCodedText(array $matches) {
+    if ($this->getEngine()->isTextMode()) {
+      $result = $matches[1];
+    } else {
+      if (count($matches) < 2 || 6 !== strlen($matches[1])) {
+        return $matches[0];
+      }
+      $match = $matches[1];
+      $fg = $this->contrastingColor($match);
+      $result = phutil_tag(
+        'tt',
+        array(
+          'class' => 'remarkup-monospaced',
+          'style' => "color: $fg; background-color: $match;",
+        ),
+        $match);
+    }
+
+    return $this->getEngine()->storeText($result);
+  }
+
+}