Personally, I'd keep Washington in the lineup over Colston. If Colston proves productive this Sunday, then you can feel safe using him as your flex for the rest of the season.
QB - Drew Brees, Tony Romo
RB - Adrian Peterson, Trent Richardson, Alfred Morris, Jacquizz Rodgers
WR - Brandon Marshall, Julio Jones, Vincent Jackson, Anquan Boldin, Golden Tate
TE - Heath Miller
DEF - Rotating
K - Justin Tucker