Its really easy,I use Dylon dye like this
http://cgi.ebay.co.uk/RED-DYLON-NO-9...QQcmdZViewItem
Put the dye and some salt in an old pan, bring to the boil, turn heat as low as possible, after a moment or two drop the wheels in and leave on the lowest heat for about 30 mins. (dont drop wheels in when boiling it could distort them). Job Done
