I don't know how it works in Washington, but, here in the Peoples Republik of Kaleefornia this is how it works.
Let's say I bought a car out of state, b/c I lived there. I just have to have had the vehicle registered in that state for 2 years I think or 12,000 miles, and I can walk into my AAA office and do a direct transfer to my beautiful state's plates with no inspection.
However, if I were you, I would just keep it registered in Colorado until you get hassled by the man. Even then, you just tell them I have a "vacation" home in Colorado and just brought the bike over here for repairs since it broke when I was on vacation. As long as your registration is current, and you have valid insurance, it should be of no consequence.
Check out the WA vehicle code, it has to be on the web. Or better yet, talk to a local dirt bike dealer, I'm sure they'll be able to help you out.