Problem deciding where to use ceiling colour and wall colour
I had this exact situation in house #1 and ended up putting wall paint on both the side and underneath the ducts.  

I first put ceiling paint on the underside, but it looked very strange from certain angles -- like lying on the bed -- where you could see all four surfaces in alternating colors.  Then I painted one small section of the duct all ceiling, and another section all wall paint, and decided the wall paint looked better overall.