In your 2nd photo, I really like the way the orange border blends with the orange and the brown border blends with the brown. I would try adding brown to the top and orange on the right side. The corners could be mitered or a blue square could be placed on each corner to continue the diagonal flow across the quilt. Then blue? for binding.
For me, the best way to decide on borders, is to lay out as many combinations as possible and take photos. There is usually one that jumps out and grabs you. :D
There are many possibilities posted here. Have fun with it!