Sorry but it is impossible.
Search the web/forum for 'yellowing amiga' and you'll find this is a common problem.
The plastic is oxydizing due to exposure to sunlight. At least that is the most common explaination. This means its yellow all the way through or at least a good portion of the plastic.
For the case this is not that much of a problem. You could opt for spraypainting it in it's original colour or somethink kinky.
For the keyboard it is a desaster since spraypainting will make you loose all the printing on the keys.
Maybe you are in luck and is the yellowing caused by nicotine (read heave smoking near the machine). This should be cleanable. If else you are out of luck. But should you happen to find a cure please notify us.
Edit: I should learn to type faster and shorter messages. Someone beat me to it :lol:
