so here's an entirely non-scientific test i just did after reading the last few posts about heel lock on the brown suede vs white gum. i just got a pair of the grey day 1010's and have a pair of the browns and the biggest difference i notice is the material used inside the shoe around the heel. on the brown pair it's is much smoother/slicker than the more coarse mesh material used on the grey. idk, like i said not really hard science but i can definitely feel a difference. i also feel like the coarse mesh material is a bit thicker if that makes any sense
anyway, food for thought