Deer apparently don't pay a lot of attention to bitterness as such. It may affect preferences when there are multiple food sources, but it's of the 'move across the street' to avoid, not 'go to the other end of the meadow'.
Thyrim (a common fungicide) and other sulphur containing compounds have some effect. Perhaps calcium poly-sulfide (Lime sulphur) should be part of the mix. Lime sulphur might not be good for the trees. One compound, rated fairly highly, was a mix of an extreme bitter agent and Thyrim.
Bordeaux's mixture (8:8:100) a mix of copper sulphate, and slaked lime, is used on both fir and spruce as a treatment for some needle cast fungi. It gives leaves an appearance like a really bad case of water spots when the dishwasher has run out of jet-dry. This demonstrates at least a lower bound on painting trees. The mix is effective through several rains. (The mix was originally applied to grapes by the road to keep passers by from eating the grapes.)
White wash (slaked lime + water + binders) was often used in dairy barns for it's antiseptic qualities. However the lime reacts with CO2 in the air to form calcium carbonate -- chalk dust or the active ingredient in Tums. However it is very white.
Latex paint is used, cut with 1 to 3 parts water (1 gallon of paint to 1 to 3 gallons water) to paint trunks of hazelnuts, apricots, and other trees that are sensitive to frost cracks.
Linseed oil is often used as a whitewash adjutant on the order of a tablespoon or two per gallon. Some recipes use a bit of soap to keep the linseed oil in suspension. Linseed oil polymerizes in sunlight and reduces the wash off rate.
Egg is used in various deer repellent recipes -- mostly for the rotten egg smell, but the protein in it may act as a binding agent too.
Milk paint is based on casein protein as the binder.
A: Find out how much latex can be diluted and still be present after, say 1 inch of rain. Some of this testing can be on smooth wood, some on say sanded core-plast, and some on actual trees.
B: Find out if latex can be used as the binder with other whitening agents. This can increase the reflectivity without increasing the longevity of the coating.
C: Test conventional ratios of bordeaux mixture, as well as mixes that contain a surplus of lime. (The reaction between copper sulphate and calcium hydroxide is significant) I have some firs heavily affected by needle cast that can be test subjects.
D: Other whitening agents: Commercial white pigment -- titanium dioxide. Probably expensive. TiO2 is harmless AFAIK. Diatomaceous earth -- silica. Might be effective against bug pests too.
E: Other carrying agents: Christmas tree growers will spray colourant on trees that are too yellow to make them greener. This is done immediately pre-harvest. It's often combined with an anti-dessiccant for better shelf life.