1. What color of feathers were worn on top of the bicorne for Generals of Brigade, Lieutenant Generals, and Marshals? I am not referring a plume but the feathers that cover the bicorne from ine side to the other. I have seen some images showing white and others black. Was it differentiated by rank?

you had technically three grades of general's rank

général de brigade
général de division
général en chef

so far as I can see - for all those three the feathers were black

for marshals it was white - I am not sure if also généraux en chefs could also have those withe feather, Rousselt on his plate Nr. 71 on generals shows however only black ones - also for a général en chef.

2. What color were the sashes worn by Generals of Brigade, Lieutenant Generals, and Marshals? Most images show alternating diagonal red and either yellow or gold for Lieutenant Generals, while Generals of Brigade's sashes appear to diagonal blue and either yellow or gold. If this is correct was the base clothe blue / red with gold embroidery? What color was the marshals' sash? Most images I have found shows white with gold diagonal stripes. . . or possibly silver with gold diagonal stripes.

The sashes were gold interwoven with cloth according to the rank.

GdB - gold with sky blue
GdD - gold with red
G en chef - gold with white

As for marshals - according to the regulations of an 12 and an 13 - gold, despite this Rousselot shows in his plate Nr. 100
always a gold with white sash.