If you read the books on pollen identification, you'll find the authors used
the same process you describe here.  So they'd be the first ones to
suggest that you do the same, especially since the plant species they
have identified most probably won't include all the species found in your
local area.  There is also the problem of variation in colour shading in
pollen pellets from the same species, depending on  time after collection,
and addition of nectar.
 
One of the good things about the Kirk book (A Colour Guide to Pollen
Loads of the Honey Bee) is that it uses a standard colour chart
(recorded as CMYK screen tint percentages).   If you really want to get
specific about pollen colours (and their variation), using such a colour
chart to compare pollen loads collected in your area is a good idea.  A
chart comparison may also help you differentiate pollen loads from
different species with very similar colours.