LED light bar mount HELP!!

Lambert is exactly right; dirt will somehow migrate under the attached magnets - I imagine through capillary-action as moisture gets underneath, dragging any adjacent particles of dirt with it.
Once the dirt has got underneath, any vibrations or sight flexing of the bodywork starts the process of damaging the paint or clear-coat. We are talking about microscopic levels really, but any damage can still be visible with the naked eye.
(If you look across your paintwork on a bright day you will see millions of little fine skate-marks, almost like extremely fine spider webs - this is where paintwork has been washed with sponges - which are known to harbour particles of grit - and is a much lesser form of the same type of damage you can expect under the magnets without the tape)

The idea of the blade-tape is that it fixes a barrier to the paint, rather like a secondary paint surface in itself.
The tape is clear, you can't see it unless you really look hard, and if you make it to the exact size of the roof bar it is unlikely it will be seen at all.

As for any fading of the paint I'd say that is the least of your worries and will be negligible.

If you are as anal as I am about washing polishing and detailing, there is a fascinating website called DETAILING WORLD that explains techniques and measures to look after your paintwork and interior, not to mention reviews on products available. Be warned however that detailing can become an addictive obsession!

Ahh ok I understand now - sounds like the tape is the easiest remedy, I'll take a look on fleabay. Thanks for all your help!

Dust and dirt gets in between the two surfaces and the minute fibration ends in marking the paint, just some clear vinyl would do the trick cut a couple of mm bigger than the bracket base