Seems that there is a round of discussion along this line each spring
and the usual problem is the ease there is in overlooking the
international nature of this list - plants that grow in Illinois may or
may not grow in London, and if they do happen to grow in both places,
they may not have the same common names.  This lead to a suggestion from
somewhere in Germany (I forget where and by whom) that the Latin names
for the plants be used to avoid any confusion.  Unfortunately, I speak
no Latin.
 
In my area (Upstate New York) the earliest source of pollen is skunk
cabbage.  I don't know what colour pollen the plant produces, but I
can describe the plant, which grows wild in moist  wooded areas.  It
flowers well before the plant itself grows (I have never actually
seen the flowers).  The plant (when it grows) resembles a head of
cabbage (again, I'm not sure if there is a realtion with the cabbage
family or not).  The plant has a most unpleasant odor resembling
that of a skunk.  Hence the plant's common name.  If you have ever
stumbled across this plant there is no mistaking it!
 
If foragers are bringing in pollen and the maples and crocus have not
yet bloomed, skunk cabbage is a good bet.  Dandelions come a few weeks
after the maples and it is at this bloom that I add honey supers.
 
Incidently, it's still hard winter here.  I envy those who are already
into the foraging season.  I expect my bees do also!