Does it really matter?
At present, recognition by one State of another State's permit is pretty much discretionary. True, one could make some constitutional arguments about why that might be improper. But until all of that gets sorted out by the courts, things are as they are.
If you're interested in carrying a gun concealed in the State of Washington, the important thing is to know what State permits are recognized. And once one has that information, he can look into getting a usable permit.
Washington (according to
http://www.handgunlaw.us/) recognizes Florida and Utah, and one can get those non-resident permits without too much hassle. And it's not hard to get a Washington non-resident permit, but you must go there to do it.
Of course, one possible reason that Washington doesn't recognize a Colorado permit is that Colorado doesn't recognize Washington's.