Question: Are there any dangers in using poly copper to deepen by horses coat color. he's black most of the year but brown in the spring.  I've heard good and bad about using copper.

Dr. KellonCompared to other species, horses are very tolerant of even high levels of trace minerals.  The main danger is that you will induce a secondary deficiency of zinc because copper competes with zinc for absorption. 
If you are going to give copper, also supplement with three times as  much zinc - e.g. 300 mg of copper and 900 mg of zinc.