Make label color and byline color match in ObjectItemList

Summary: We're using two different greys here.

Test Plan:
Inspect both elements, now serving the same grey.

{F253237}

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11009