All usb ports are charging ports aka 2.5W. There are not any marked YELLOW Always on USB ports that "charge" whether the system is on or off.
There is a setting in CMOS USB PowerShare This option allows you to charge the external devices, such as mobile phones, music player. This option is disabled by default.
This Dell article, www.dell.com/.../usb-powershare-feature, indicates that the PowerShare ports are marked with a lightning bolt USB symbol but on the XPS 8910 I found no such symbols next to the USB ports although the PowerShare option is in the BIOS which makes you wonder if all ports are PowerShare or only certain ones. Also all the front USB 3.0 ports are the same color (black) and the back USB 3.0 ports and USB 3.1 port the same color (blue).