The puzzle

Four angels sat on the Christmas tree amidst other ornaments. Two had blue halos and two yellow. However, none of them could see above his head.

Angel A sat on the top branch and could see the angels B and C, who sat below him. Angel B, could see angel C who sat on the lower branch. And angel D stood at the base of the tree obscured from view by a thicket of branches, so no one could see him and he could not see anyone either.

Which one of them could be the first to guess the color of his halo and speak it out loud for all other angels to hear?

From Brainden.com