I didn't pursue information on Colorado Lake much further. I had a rather ugly situation come about when we tried to visit Fisher Island Pond east of Corvallis. I did a bit of research on how to access it via Ireland Lane (the "no trespassing" route), and a guy with two chained/aggressive dogs came out cussing and yelling. We turned around once my dad got a few choice words in. Situation could have turned physical... the guy definitely did not want to discuss/help/point us in the right direction.
So, dealing with potentially private ponds has me fairly spooked for the time being. I did talk to a fish biologist at ODFW; he gave us some recommendations for other ponds he knows to be open. He also encouraged me to talk with the Benton County Assessment Office. However, I'm not incredibly eager to fish ponds or have the Ireland Lane issue corrected. I'm aware that there may be an easier access route to Fisher Island right along 34... if I decide to try again.
Well, thanks for the input everybody! I'm sure reaching Colorado Lake won't be so problematic, but it'll be a while before I look into it.