If you are Walla Walla to Spokane... is there a reason you want to go to Colfax? I've got a bunch of suggestions for you. My back yard and I can get you down some lesser known awesome roads with lots of Palouse views.
Where in Spokane do you want to end up? Spokane, South or North, or Spokane Valley?
Regarding Omak... I would skip 174 and take 21 out of Wilbur to the Keller Ferry (free ferry). Great park there with a gazebo for lunch if interested. You can take either Manilla Creek Road to Peter Dan road and over to 155 just north of Elmer City... or, if you have the time, a little further north on 21 is Cache Creek Road and that will take you over to 155 as well and dump you out about Nespelem.