diff --git a/lib/prawn/graphics/color.rb b/lib/prawn/graphics/color.rb
index e22886564..cebcdf7a6 100644
--- a/lib/prawn/graphics/color.rb
+++ b/lib/prawn/graphics/color.rb
@@ -94,7 +94,7 @@ def process_color(*color)
       def color_type(color)
         case color
         when String
-          if color =~ /\A[0-F]{6}\z/i
+          if color =~ /\A[0-9|A-F]{6}\z/i
             :RGB
           else
             raise ArgumentError, "Unknown type of color: #{color.inspect}"
diff --git a/spec/prawn/graphics_spec.rb b/spec/prawn/graphics_spec.rb
index db9c878d7..827c8a0c5 100644
--- a/spec/prawn/graphics_spec.rb
+++ b/spec/prawn/graphics_spec.rb
@@ -245,6 +245,10 @@
       expect { pdf.fill_color 'zcff00' }.to raise_error(ArgumentError)
     end
 
+    it 'raises an error for a color string with invalid characters' do
+      expect { pdf.fill_color 'f0f0f?' }.to raise_error(ArgumentError)
+    end
+
     it 'resets the colors on each new page if they have been defined' do
       pdf.fill_color 'ccff00'