Custom dropdown attribute sort by option is not showing proper result on category page in magento 2

I am using magento 2.2.6 and create custom product dropdown attribute called patter_name and i wanted this to use for product sort by on category page.

but when i sort by this attribute “patter_name“, result is not coming proper.

below is the result

Color Blue Color Green Color Apple

Expected result is

Color Apple Color Blue Color Green

Please help me