black. spray it all black. it actually makes it look higher since it visually 'fades' away.

Unfortunately, I don't think we can paint the silver flexible ducts.

Why can’t you paint the flexible ducts? We painted our previous ceiling a greyish blue called blueberry. It was awesome. We spray painted everything Including ductwork, pipes, electrical. I would not suggest white since you’ll see lots of shadows. You want all the different surfaces to disappear.

sure you can.

Furnace ducts can be painted, but using the correct kind of paint is necessary for the best results.

Paint. Oil-based paint that is rated for use on metal works best on furnace ducts. Avoid latex or regular paint because it will not adhere properly, even on top of a coat of primer. Use a paint roller in visible spots for a smooth finish and a paintbrush for other areas.

I don't think white looks as good