lowercase the color

That just move color to lowercase. There is no standard around it but it looks like it tends to be more lowercase than upercase :

• https://www.drupal.org/project/drupal/issues/1360790
• https://github.com/prettier/prettier/issues/2139#issuecomment-309569068
• https://bugs.chromium.org/p/chromium/issues/detail?id=577505#c19