I have never seen touch up paint for the roll over hoops being advertised. I have read threads where members have taken then covers off and had them resprayed, this could be an alternative option for you may be
Careful masking and respray is the way to go, take a few cans of silver paint for various cars from your friendly local auto shop and choose nearest match or go for body colour Black Sapphire. All touch up paints tend to stand out like a sore thumb IMHO. Cheers Baz