We've only just been introduced to the Doctor's flash (flashing) new Sonic Screwdriver so any chance to have a closer look is appreciated. Enter some of the original production designs for the device, complete with colour palettes and... ooh, what's that...?
We haven't seen the Sonic glowing red at any point in the series yet but in the drawings there are what look like red lights along the length of the top half and at the tip. Could this be the fabled 'red setting' we've heard about...?
We first came across a Sonic Screwdriver with this special mode in Forest of the Dead when River Song was able to use the red setting on hers (given to her by the Doctor in a previous meeting) to prevent interference from the sentient computer Doctor Moon. In series seven episode Cold War, meanwhile, the diode at the tip of the Eleventh Doctor's Sonic glowed red when he threatened to blow up the submarine.
So while it's not completely clear what the red setting does and how far its apparent special abilities range, it at least looks as if, in time, we might find out...