Climbed with Flat Top, which added a lot to my day actually, almost 6 more miles and 1500 vertical extra from the saddle! Derby is well guarded by cliffs from this side but I found a steep class 2+ gully weakness above a tarn above Hooper Lake. It actually works pretty well and has a game or climbers trail through parts of it. Its a little loose, just have to take it slow as a fall would be potentially catastrophic in the wrong place. If Derby is your only objective, use a different route.
Climbed Derby as part of an weekend loop from Trappers Lake. Little Trappers Lake, Chinese Wall, Derby Peak, Deer Lake, Upper Island Lake, Wall Lake, Trappers Peak down to Trappers Lake. Amazing views.
The Flat Tops are the true Hidden Gem of Colorado.