[EBB Sightings] White hummer?

In Emeryville near the UA Theater we saw a hummingbird that looks very 
white (not albino), with just a faint bit of color on the sides and a 
little black edging on the underside of the tail.  The back and head 
look completely white, the bill is dark.  We've gone through all our 
bird books and haven't found anything that looks this light colored all 
over, but it seems much smaller than the Anna's hummer I have nesting in 
my yard.  Could it be a juvenile Anna's?  The other hummer that looked 
approximately the same in the bird book is a juvenile Costa's since it's 
so much smaller than an Anna's but that would be pretty rare here.  If 
anyone else lives near Emeryville and can take a look, maybe you can ID 
better than me!

My partner has seen this bird every day around 1-2pm when she walks to 
lunch and I saw it there in that time period as well. Not sure if it's 
around at other times of day, but it's possible.  Exact location:  in 
the parking lot between the UA Emeryville Theater and the Marketplace 
there is a long walkway lined with trees.  The hummer is usually singing 
in these trees which fortunately don't have their leaves yet.  We 
watched it for about 10 minutes before it finally flew off into the 
(leafed out) trees on the the other side of the lot and we couldn't 
refind him.  Not sure if the rain will deter it or not!