Yellow and white (or two yellows) arranged horizontally, mean that the signal may be passed
only if the following points are set for the headshunt (or other specified route). If the route is set towards the main line, then the signal cannot be passed unless it is displaying two (diagonal) white lights. These 'yellow shunts' allow shunting moves to take place, in theory, in and out of a headshunt repeatedly, without the involvement of a signalman. They're very rare now though, with two normal GPLs (i.e. position light signal, as above) being provided in modern schemes - one for moves towards the headhunt, and another in the opposite direction. Usually both can be left 'off' to allow the shunter to do exactly as he wishes
.
Two whites, by the way, means 'proceed with caution as far as the line is clear' - doesn't mean there's not a train before the next signal! The arrow on the signal identification plate just shows which line the signal refers to, if it's situated on the ground between two lines.