I've never heard this before, it is possible I suppose as you can get LEDs that will display more than one colour but I would think they would be more expensive than standard LEDs and therefore any PC company looking to save costs would not use them unless really necessary.
I realise this might seem like a strange thing to say as the difference is likely to be only a few pence but if you sell as many PCs as Dell then it would soon add up.
I also think that changing the colours would not be that simple as the colours would have a particular function and as they are hard wired it probably couldn't be changed.
Hopefully someone that has one of these machines can advise on to whether this is possible.
Personally I would think not but perhaps you could tell us where you saw this info on changing the LED colours and this may give us somewhere to start.