I did some playing around this morning and it looks like the API is just not accepting WHITE 0xffffff just doesn't work at all. If you add 95% opacity then it does work and in all cases for me looks very white:
Works:
dialog.messageColor = "0xF2FFFFFF"
Doesn't work:
dialog.messageColor = "0xFFFFFFFF"
Someone on the scene graph team might wanna reproduce / add to a defect queue.