Hi, Billie. Is it a mop-top variety — with big round flowerheads like you often see in the color blue — or a paniculata type like the one in the photo? All of the hydrangea flowers will turn tan and brown as they age each season (though not right away!), whether in sun or shadier spots, and will bloom more heavily in sun. Yes, you can put it in a barrel if the pot is big enough to insulate the roots well in winter, and big enough for the plant in general. It would be best to get a dwarf-ish variety instead of one of the ones that wants to be very large if you plan to confine it to a pot, of course.

Sounds as if you have it nailed, Susan. Love growing garlic!