Incorporated in Colorado, doing work in California
Obviously this is a very grey area. The fact that one of the members lives in California by itself doesn't immediately create strong nexus in that state. However maintaining an office (even a home office) in California would already be a sign that the company is doing business in that state, and might require local registration.
Also, having a client in California wouldn't immediately require registering there, however taking all the factors into consideration, and given the nature of the involvement with that client you might be in a position where such registration (foreign entity) would be required, according to the State.
Bottom line is, its quite murky and in order to get a solid yes/no answer you probably should consult a CA licensed attorney.