Using [UIColor colorWithRed:green:blue:alpha:] doesn't work with UITableView seperatorColor?

I'm trying to configure a dark gray seperator color. Why does the following do nothing?

self.tableView.seperatorStyle = UITableViewCellSeperatorStyleSingleLine;
self.tableView.seperatorColor = [UIColor colorWithRed: 127 green:127 blue:127 alpha:1];

returns a table with no seperators at all.

As soon as I use [UIColor blackColor] then I get seperators just fine. What's the deal?