Vienna's U1 is designated in red. I especially like the fact that they use the line color in many places, such as this stairway handrail.
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/94611718@N00">Andrew Nash</a> from Vienna, Austria
CC BY-SA 2.0
<p><a rel="nofollow" class="external text" href="https://www.flickr.com/photos/andynash/8220852717/">Red Handrail U1 Vienna</a>
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<ul><li>Uploaded by <a href="//commons.wikimedia.org/wiki/User:Tokfo" title="User:Tokfo">Tokfo</a></li></ul>