The first would be to use Easy Colour which is a wipe on coloured finishing cream. The alternative is to use the pigment products which would have been used to colour the leather in the first place.

Permanent markers are ok on wood but should not be used on leather. Do not use any sort of furniture or shoe polish as if this does not work it may be impossible to fix.

There is so much misinfomation about leather on the internet, I have just read on another forum that using scotch tape will remove glue and biro from leather. It will but it will also leave this sort of damage behind.

Please be careful what advise you take, always consult a trained professional before trying to fix anyhting on leather as it can so easily go wrong with homespun remedies.
References :
http://www.LTTsolutions.net
Leather care consultants
Industry recognised Training company for leather technicians and restoration experts.