Add unit test for non hexadecimal string

spec/unit/color_spec.rb

@@ -37,7 +37,7 @@ module Rainbow
         specify { expect(subject.b).to eq(112) }
       end
 
-      context "when single string given" do
+      context "when single hexadecimal string given" do
         let(:values) { ['#deadcc'] }
 
         it { should be_instance_of(Color::RGB) }
@@ -48,6 +48,14 @@ module Rainbow
         specify { expect(subject.b).to eq(204) }
       end
 
+      context "when single non hexadecimal string given" do
+        let(:values) { ['green'] }
+
+        it 'raises ArgumentError' do
+          expect { subject }.to raise_error(ArgumentError)
+        end
+      end
+
       context "when 2 elements" do
         let(:values) { [1, 2] }