As the OSD have two blue corners, I guess progress bar could be nice in blue too, but that way the volume ctrl would be blue too.
Any way, I like both, volume ctrl and progress bar in grey or blue, as I like SNR bar in red or blue, red in this bar is always ok in all OSD, and blue, in blue/black/grey OSD, is ok too.
Talking about volume ctrl, I guess volume ctrl symbol in "Rihanna 1" and "Rihanna 2" (that is white) could have best contrast, in blue background vol. ctrl, if was in black or at least with a black thin border, this is just my humble opinion.