diff --git a/lib/rainbow/color.rb b/lib/rainbow/color.rb
index 4140c0c..8ce5785 100644
--- a/lib/rainbow/color.rb
+++ b/lib/rainbow/color.rb
@@ -36,6 +36,11 @@ def self.build(ground, values)
     end
 
     def self.parse_hex_color(hex)
+      unless hex =~ /^#?[a-f0-9]{6}/i
+        raise ArgumentError,
+          "Invalid hexadecimal RGB triplet. Valid format: /^#?[a-f0-9]{6}/i"
+      end
+
       hex = hex.sub(/^#/, '')
       r   = hex[0..1].to_i(16)
       g   = hex[2..3].to_i(16)