Not only does fast ball match the gold with the little yellow lightning bolt and details on the sides (I'd post a picture but I'm on my phone), but with a fast ball you can appreciate the irony due to Steelix being hell'a slow.I beg to differ, Fast Ball is not as aesthetically pleasing as is Safari Ball on shiny Mega Steelix imo.
Safari ball is good on shiny onix which is a more greenish sort of colour. This is my opinion of course, so I guess anything goes really
EDIT: for further evidence, just look at other pokemon that appreciate safari ball, such as shiny Machamp or shiny scizor. All quite on the green side as opposed to yellow/gold