Idaho is a good place to get a color-phase bear, especially cinnamon phase. You might also try Colorado and New Mexico and Arizona. Those are also good states in which to find various color phases. But, if you are adamant about Idaho only, I'd look to the drainages within the Salmon River system.
I have never seen a non-black color phase bear in the Salmon River drainage and can easily document having hunted there for far more than 100 days.
The Boise River drainage has a high percentage of non-black color phase. I would guess-ta-mate there is a 30-40% chance of getting a brown or cinnamon and only saw the one blonde. I've seen a blonde phase in Hells Canyon and some other non-black but seems to have a higher percentage of black.
Whichever guide you select should have data and probability information. I've only shot 6 bears and only taken up actually hunting them recently. Most of my observations have been during the spring whitewater rafting and fall hunting seasons.