Any time I've grown CP, the shoulders are always green when ripe. The softness is how I judge.

I pick by softness too, and my CPs generally do have green shoulders too---that is just one of their characteristics. There is a difference in the color they are when I pick them versus when I eat them. I generally pick them when the blossom end breaks color and begins to turn the deep brick reddish-purple. At that point the shoulders still are really green, and the fruit does not yet feel as soft as a true ripe tomato feels. I let them sit on the counters a few days to finish ripening, and most of the green shoulders fade a bit before they ripen. Some fruit have shoulders that stay greener than others. After growing CP for a long time, I feel like the color variation is normal and occurs all season long. The hotter we get, the more the green shoulders fade to purplish green. Early in the season they seem to hold their greenness longer.

Here's some photos of CP from Tatiana's Tomatobase for comparison's sake, but keep in mind that growing conditions can determine how much green even a ripe one has. In addition to the one photo at the top of the page, if you scroll down to the Gallery at the bottom of the page, you'll see another 12 or 14 photos of Cherokee Purple Fruit and will note a variation in the degree of greenness left on various ones. That's exactly what I see---a lot of variation, so the softness test is how you know they're ripe. If they're too hard, they aren't fully ripe yet.

Cherokee Purple Page at Tatiana's Tomatobase