Try a new font maybe? Look into GUI skins... file:///Applications/Unity/Unity.app/Contents/Documentation/Documentation/ScriptReference/GUISkin.html

I've tried the handful of fonts and they all end up this way. Moreover, this is the only OnGUI() call that seems to cause this. The other 4 calls on this screen work correctly (showing white text).

Is there any way you'd be able to share the OnGUI code where this is being called, or other aspects of the project that might help get to the root of the problem?

The color may be caused if you have set the font material color to be black; in which case even if the GUISkin is set to white, the white -or unaltered -version of black is still going to be black.

Absolutely:

void OnGUI ()
{
    float height = 20F;
    for(int i = 0; i < _MissionInfo.MissionTasks.Count; i++)
    {
       GUIStyle style;
       if(((MissionInfo.MissionTask)_MissionInfo.MissionTasks[i]).Completed)
         style = MissionCompletedStyle;
       else
         style = MissionNotCompletedStyle;
       GUI.Label(new Rect(Screen.width-150, height, 150, 25), ((MissionInfo.MissionTask)_MissionInfo.MissionTasks[i]).FriendlyText, style);
       height = height + 25;
    }
}