The grouting between your ceramic tiles will often become discoloured with age.
You may also get black mildew forming in damp areas like bathrooms. To get them looking good again you can try a few different things. Re grouting them completely wil get them looking good as new however re grouting will require quite some time and effort. Before attempting that you may want to consider several quicker options.
How to re-grout
The first option would be to try scrubbing them with bleach and hard tooth brush. Can be affective but is still quite laborious.
Alternatively you can try a steam cleaner. Several diy steamers come with attachments with fine nozzles suitable for this.
Check out the Earlex web site they do a range of steam cleaning kits www.earlex.co.uk
A quick option is to try a grout pen. This is basically a pen that you simply draw on. It's cheap and easy. The success depends on how bad the tiles have become. Ronseal make pens in three colours check them out at :http://www.ronseal.co.uk/home/kitchen-and-bathroom/one-coat-grout-pen/
NB. If you fancy changing the colour of the tile completely Ronseal also make special tile paint. Simply paint over the whole lot !!