The wizard in yellow robes appears in Runespan. You can hand any type of rune to the yellow wizard but will be provided with bonus XP If you give them the rune they are asking for. The XP for runes not specified by the wizard is based on the type of rune and your Runecrafting level. He will take up to 10 of the given rune.
You will need to be on the same floor for at least 10 minutes before you will be notified that one of the 5 wizards is stranded. Note that there are 5 wizards on each floor and the stranded wizard (i.e. the one in yellow) is not the same for every player.
Search for the yellow wizard and speak to them. If you spot a blue wizard and speak to them they will help you track down the yellow wizards.
Below level 77 it is best to give the wizard the highest rune you have; regardless of what he asks for. If he happens to ask for the highest rune you have it is more rewarding to give him your second best rune. (This will hopefully be fixed soon)
At level 77+ it is better to give the requested rune because the bonus xp is always larger. If you do not have the runes he asks for, give him the highest runes you have.
You will not be able to hand runes in more than once per 10 minutes.
All of the wizards, including the "stranded wizard" will happily teleport you back to the Runecrafting Guild entrance if you need to.