If they are aluminum, a bit of wet and dry will get it looking like new again

So, to follow up:
I went out (between heavy showers) and gave one side a quick rub with some T-cut on a rag. The marks were easily removed and so far haven't returned. I would suggest it's the aluminium oxidising differently along its length. Why tiger stripes, I don't know. Maybe a result of the manufacturing process where pressure was applied during forming/cooling.
I wouldn't use anything too abrasive. Any roughening or texture will trap dirt.

Thanks for the tip I'll give it a go when it stops raining!!

Although some have suggested they don't like steps I have had many comments on how the steps make the car look lower and wider although I bought them to help put bikes on the roof mounted rack...I'm not too concerned about ground clearance up and down the motorway all day...have had my second puncture in 2 weeks today though so i may as well have been off roading! SHINY BLACK BEAUTY - FL2GS AUTO WITH A FEW OPTIONS...AFTERMARKET (SO FULL!!) ALPACA LEATHER, LEATHER WHEEL, 18" ALLOYS, PHILIPS BRIGHT HEADLAMP BULBS, PRIVACY GLASS, CHROME MIRRORS, CLEAR INDICATORS, THULE ROOF BARS, FL2 STICKER, WHAT TO GET NEXT?

Do you have to take the steps off to get the jack on ?

I personally think FL2's look plain and dull without steps or tubes !

No need to remove them to get to the jacking point, bad news was tyre has had it have managed to pick up a 2 inch piece of steel through it, lucky not to get a blowout really! Anyone know why the steel spare and tyre is limited to 50mph it looks like a normal 17 inch tyre to me?

Just waiting for the new tyre to be delivered although I was tempted by a lovely set of 20" khan black and silver wheels that are on ebay at the moment but the missus seemed to think the money was better spent on bedroom furniture boooh!! SHINY BLACK BEAUTY - FL2GS AUTO WITH A FEW OPTIONS...AFTERMARKET (SO FULL!!) ALPACA LEATHER, LEATHER WHEEL, 18" ALLOYS, PHILIPS BRIGHT HEADLAMP BULBS, PRIVACY GLASS, CHROME MIRRORS, CLEAR INDICATORS, THULE ROOF BARS, FL2 STICKER, WHAT TO GET NEXT?