diff --git a/src/color.js b/src/color.js
index 6552e3d..378a8cd 100644
--- a/src/color.js
+++ b/src/color.js
@@ -9,12 +9,12 @@ var reI = "\\s*([+-]?\\d+)\\s*",
     reN = "\\s*([+-]?(?:\\d*\\.)?\\d+(?:[eE][+-]?\\d+)?)\\s*",
     reP = "\\s*([+-]?(?:\\d*\\.)?\\d+(?:[eE][+-]?\\d+)?)%\\s*",
     reHex = /^#([0-9a-f]{3,8})$/,
-    reRgbInteger = new RegExp("^rgb\\(" + [reI, reI, reI] + "\\)$"),
-    reRgbPercent = new RegExp("^rgb\\(" + [reP, reP, reP] + "\\)$"),
-    reRgbaInteger = new RegExp("^rgba\\(" + [reI, reI, reI, reN] + "\\)$"),
-    reRgbaPercent = new RegExp("^rgba\\(" + [reP, reP, reP, reN] + "\\)$"),
-    reHslPercent = new RegExp("^hsl\\(" + [reN, reP, reP] + "\\)$"),
-    reHslaPercent = new RegExp("^hsla\\(" + [reN, reP, reP, reN] + "\\)$");
+    reRgbInteger = new RegExp(`^rgb\\(${reI},${reI},${reI}\\)$`),
+    reRgbPercent = new RegExp(`^rgb\\(${reP},${reP},${reP}\\)$`),
+    reRgbaInteger = new RegExp(`^rgba\\(${reI},${reI},${reI},${reN}\\)$`),
+    reRgbaPercent = new RegExp(`^rgba\\(${reP},${reP},${reP},${reN}\\)$`),
+    reHslPercent = new RegExp(`^hsl\\(${reN},${reP},${reP}\\)$`),
+    reHslaPercent = new RegExp(`^hsla\\(${reN},${reP},${reP},${reN}\\)$`);
 
 var named = {
   aliceblue: 0xf0f8ff,
@@ -176,6 +176,7 @@ define(Color, color, {
   },
   hex: color_formatHex, // Deprecated! Use color.formatHex.
   formatHex: color_formatHex,
+  formatHex8: color_formatHex8,
   formatHsl: color_formatHsl,
   formatRgb: color_formatRgb,
   toString: color_formatRgb
@@ -185,6 +186,10 @@ function color_formatHex() {
   return this.rgb().formatHex();
 }
 
+function color_formatHex8() {
+  return this.rgb().formatHex8();
+}
+
 function color_formatHsl() {
   return hslConvert(this).formatHsl();
 }
@@ -262,12 +267,17 @@ define(Rgb, rgb, extend(Color, {
   },
   hex: rgb_formatHex, // Deprecated! Use color.formatHex.
   formatHex: rgb_formatHex,
+  formatHex8: rgb_formatHex8,
   formatRgb: rgb_formatRgb,
   toString: rgb_formatRgb
 }));
 
 function rgb_formatHex() {
-  return "#" + hex(this.r) + hex(this.g) + hex(this.b);
+  return `#${hex(this.r)}${hex(this.g)}${hex(this.b)}`;
+}
+
+function rgb_formatHex8() {
+  return `#${hex(this.r)}${hex(this.g)}${hex(this.b)}${hex((isNaN(this.opacity) ? 1 : this.opacity) * 255)}`;
 }
 
 function rgb_formatRgb() {
diff --git a/test/rgb-test.js b/test/rgb-test.js
index 158a841..94a347c 100644
--- a/test/rgb-test.js
+++ b/test/rgb-test.js
@@ -46,6 +46,13 @@ it("rgb.formatHex() formats as #rrggbb", () => {
   assert.strictEqual(rgb("hsla(60, 100%, 20%, 0.4)").formatHex(), "#666600");
 });
 
+it("rgb.formatHex8() formats as #rrggbbaa", () => {
+  assert.strictEqual(rgb("#abcdef").formatHex8(), "#abcdefff");
+  assert.strictEqual(rgb("hsl(60, 100%, 20%)").formatHex8(), "#666600ff");
+  assert.strictEqual(rgb("rgba(12%, 34%, 56%, 0.4)").formatHex8(), "#1f578f66");
+  assert.strictEqual(rgb("hsla(60, 100%, 20%, 0.4)").formatHex8(), "#66660066");
+});
+
 it("rgb.hex() is an alias for rgb.formatHex()", () => {
   assert.strictEqual(color.prototype.hex, color.prototype.formatHex);
   assert.strictEqual(rgb.prototype.hex, rgb.prototype.formatHex);